The dyeability of polypropylene fibres modified by two nitrogen polymeric additives containing dye sites of different basicity and accessibility was investigated using an acid dye. The thermodynamic and kinetic parameters of dyeing under infinite bath conditions were determined for four aqueous dye solutions. It is postulated that ionized molecules of acid dyes diffuse within the fiber after activation of a dye site by a suitable agent. In addition, color yield and color fastness of various dyes have also been studied.