This invention relates to a process for the preparation of a polycaprolactam which can be dyed with basic dyes and worked up from the melt to form products such as fibers or foils. The polycaprolactams contain structural elements of the formulae ##SPC1##WhereinM denotes an alkali metal andR denotes an alkylene group containing 4 - 12 carbon atoms.The process of the production of the polycaprolactams comprises precondensing the alkali metal salt of a dialkyl ester of 5-sulphoisophthalic acid in aqueous solution with an excess of hexamethylene diamine and adding caprolactam and an equivalent quantity based on the hexamethylene diamine excess of an aliphatic dicarboxylic acid to the resulting aqueous precondensate and polycondensing the mixture.