A cadmium salt of a light-sensitive diazo resin is obtained from sulphuric acid solution by (a) treating the solution with a cadmium salt, (b) separating the precipitated cadmium salt and treating it with BaCl2 in excess of that required to precipitate the sulphate ion remaining and (c) extracting the resin salt from the resulting mixture of solids. The resin solution is obtained by adding an intimate mixture of a diazo aniline salt and an aldehyde to concentrated sulphuric acid. In the Example a mixture of a p-diazodiphenylamine-zinc chloride double salt and paraformaldehyde is added to conc. H2SO4 below 5 DEG C. The solution is mixed with ice and excess CdCl2, then isopropanol is added and the slurry filtered. The solids are slurried with water and BaCl2, then filtered and twice extracted with HF solution and once with water. The combined filtrates are treated with excess CdCl2, then with isopropanol and the resultant slurry filtered.