In this article we have evaluated some of the factors (pH, solvent composition and dye structure) that influence the dyeing of hair with dyes from direct semipermanent categories. These dyes, in fact, have been used more and more in cosmetic industry over the last years, due to their lesser aggressiveness vs. hair fibres. Two different dye classes are investigated: phenylene diamine (HC), well known for their low molecular weight and their small size, and a series of Basic dyes, characterized by the same ionic charge (positive) but with different sizes and different polar groups in the molecule. pH and dyebath composition result critical factors controlling the diffusion and adsorption process.