Metal complex polyazo dyes having, as a coupling component radical, the radical of a coupling component K, being the product of the condensation, under acidic conditions, of the formaldehyde with a compound or mixture of compounds of formula (a), ##STR1## AND/OR A COMPOUND OR MIXTURE OF COMPOUNDS OF FORMULA (B), ##STR2## WHEREIN EACH R.sub.1 , R.sub.2, R.sub.3 and R.sub.4 is independently is hydrogen, halogen, hydroxy or optionally substituted alkyl or alkoxy, andM is hydrogen or a non-chromophoric cation,The mol ratio of formaldehyde to the other compound(s) (a) and/or (b) being at least 0.6:1 and at least 50 mol % of the other compound(s) (a) and/or (b) containing a --COOM group, and, as diazo or bisdiazo component radical, a radical D- or -D- where D contains at least one metallized azo group, their production and their use for the dyeing of anionic dyeable substrates, particularly leather. The dyes have good levelling capacity and the obtained dyeings possess notable fastness to light.