In a continuous process for transversely stretching a travelling continuous film of a thermoplastic synthetic organic polymeric material, the advancing film, during a transverse stretching operation, is treated on one side only by direct contact with a hot vapour or a hot liquid. The treating agent may be steam, hot water or a hot organic liquid containing water, such as hot aqueous ethyl alcohol. As shown in Figs. 1 and 3, a continuous film 1 is led in a taut condition over the open top of a tank 2 which gradually widens in the direction of travel of the film and is provided with a horizontal rim 3. The tank is filled with a hot treating liquid 4 to a level which, when the liquid is still, is slightly below the brim. Two series of clamps 5, which hold the edges of the film, are attached to the chains of a stretching frame and are conveyed along the rim 3 of the tank 2 and follow its widening, thereby transversely stretching the film. The film is in con <PICT:0805733/IV (a)/1> <PICT:0805733/IV (a)/2> <PICT:0805733/IV (a)/3> tact with the rim 3, so that it seals the mouth of the tank and the lateral edges of the film are kept out of contact with the treating liquid. During this stretching operation the hot treating liquid is made to wash up against the underside of the film by pumping the liquid under pressure through pipes 6 (Figs. 2 and 3) immersed in the liquid and provided at their upper sides with openings 7. A stream of liquid is thus ejected through each opening 7 through the main body of the liquid, as shown by the arrows in Fig. 3, and impinges on to the underside of the film, leaving air pockets 12 in the main body of the liquid. An analogous method is used when the underside of the film is treated with a hot vapour. The tank 2 is provided with overflow ducts which conduct the treating liquid to an external heater which re-adjusts it to the desired temperature and from which it is returned to the tank by means of a circulating pump via the pipes 6. Advantageously the film is first led, while already clamped to the chains of the stretching device, but without being stretched, through the zone 8 (Fig. 1) of the stretching frame in which the chains travel parallel to one another before entering section 9 of the stretching frame, where the clamp chains diverge. The film is thus heated or softened to the desired degree before being stretched. It is also preferable not to release the film from the clamps 5 immediately after the stretching operation, but to pass it over a further portion 10 of the tank while the film is still under tension but the chains again travel parallel to one another. The film may thus be cooled by contacting its under-surface with a liquid at a lower temperature before it is released from the clamps. Varied degrees of softness may be imparted to the film while held by the clamps before, during, or after the stretching operation by treating different portions of the under-surface of the advancing film simultaneously with liquid or vapour at different temperatures, the tank 2 being divided into several compartments by transverse partitions. The upper surface of the film, particularly its marginal areas, may be treated at the same time as the undersurface with, for example, infra-red rays, or cool air, or may be thermally insulated. In an example, a continuously travelling film of a polyterephthalic acid ester of glycol of 250 microns thickness and 380 mm. width, formed by melt extrusion, is introduced into a stretching frame having an inside transverse dimension of 340 mm. at the entrance and a rate of film advance of 2 metres per minute. For the first 800 mm. the clamps of the frame are conveyed along parallel paths of travel, and in this zone water at 88 DEG C. is washed against the underside of the film. Subsequently the clamps are led along diverging tracks while advancing the film 1900 mm. the angle of divergence between each track and the median line between the tracks being about 12 degrees, and the underside of the film being washed with water at 78 DEG C. The film is finally led, while still clamped, a distance of 800 mm., the path of travel of the clamps being parallel and the transverse distance between the clamps being 1150 mm. During this final portion of its path of travel the film is not washed with water but is allowed to cool down until it is released from the clamps and leaves the stretching frame. In the second example a plasticised film of a mixed polyamide, obtained by polycondensation of 4 parts by weight of caprolactam and 6 parts by weight of the adipic acid salt of diamino hexane, is transversely stretched in a similar manner.