Azo dyes, chromium compounds of.--Chromiferous azo dyestuffs are obtained from a mixture of chromable azo dyestuffs, of which at least one is derived from diazotized 1 : 2-aminonaphthol-4-sulphonic acid or its nitrated derivative and a - or b -naphthol, by treating the mixture in boiling aqueous solution with an agent yielding chromium in presence of a proportion of alkali metal ions insufficient to salt out the dyestuffs until the solution no longer contains any dyestuff capable of being salted out, and thereafter, if necessary after filtration, evaporating the solution to dryness at a lower temperature, preferably under reduced pressure. Chromium oxalate or a mixture of chromium sulphate or sulphite with chromium oxalate are particularly suitable as chroming agents. Alternatively, one of the azo dyestuffs of the mixture may be replaced by a chromium compound thereof, this compound serving also as the agent yielding chromium. The products are suitable for dyeing animal fibres, including leather and for use in lacquers and varnishes and in printing. Dyeing may be effected in weak acid baths with development by increasing the acidity of the baths as described in Specification 364,148. The following examples are specified: (1) An aqueous suspension of a mixture in specified proportions of the dyestuff from nitrated diazotized 1 : 2-aminonaphthol-4-sulphonic acid and b -naphthol, the dyestuff 1 : 2 - aminonaphthol - 4 - sulphonic acid --> a -naphthol, and the dyestuff 4-nitro-2-aminophenol-6-sulphonic acid --> 1-phenyl-3-methyl-5-pyrazolone is boiled for 2-3 hrs., while stirring, with the addition of caustic soda, sodium formate, chromium bisulphite and chromium oxalate; the product, isolated by evaporation to dryness under reduced pressure, dyes wool from an acid bath fast pure black tints. (2) An aqueous suspension of the same mixture of dyestuffs is boiled for 2-3 hrs., while stirring, with the addition of sodium formate, sodium sulphite, and a solution of chromium hydroxide in a mixture of oxalic and hydrofluoric acids; the product, isolated as in (1), dyes wool from an acid bath fast pure black tints; by varying the proportions in the dyestuff mixture more reddish dyeings may be obtained. (3) An aqueous suspension of dyestuffs as in (1) is boiled for 1\ba1/2\be-2 hrs., with the addition of sodium formate, sodium sulphite, and a solution of chromium hydroxide in a mixture of oxalic acid and sulphuric or hydrochloric acid; the product, isolated as in (1), dyes wool from an acid bath fast pure black tints. (4) A mixture in specified proportions of the complex chromium compound obtained according to example 3 of the parent Specification and the azo dyestuff from nitrated diazotized 1 : 2-aminonaphthol-4-sulphonic acid and b -naphthol is boiled in aqueous solution for 4 hrs. under reflux; the product, isolated as in (1), dyes wool from an acid bath fast black tints. In an example of the dyeing process wool is treated in a dyebath with a formic acid solution at 85-90 DEG C., more formic acid and the dyestuff product of example (1) are added, dyeing is continued at the same temperature for 10 mins. and at the boil for a further 20 mins., dilute sulphuric acid is added during the following 30 mins. and the dyeing is developed by boiling for 1 hr. to obtain a fast black. The Specification as open to inspection under Sect. 91 comprises also an example in which wool is dyed fast deep black tints by treatment in a formic acid bath at 90-100 DEG C. containing the complex chromium compound obtained according to example 3 of the parent Specification and the dyestuff from nitrated diazotized 1 : 2 - aminonaphthol - 4 - sulphonic acid and a -naphthol and development by addition of sulphuric acid to the bath. This subject-matter does not appear in the Specification as accepted.