Compositions including polycarbodiimide and derivatives thereof together with at least one additional active selected from one or more a carboxylic acid compound chosen from fatty acids, their salts, and mixtures thereof, a polycarboxylic acid compound other than one that is chosen from fatty acids, their salts, and mixtures thereof, and a cationic polymer to enhance the quality of the keratinous substrates. The present invention relates to a cosmetic treatment and process for treating keratinous materials, in particular for hair-care and hair-styling, wherein the composition can include the polycarbodiimide and at least one additional active in amounts sufficient to impart hydrophobicity to keratinous substrates, including hair fibers, upon application thereto. The present invention, in particular, relates to a hair treatment composition and process for enhancing durable non-permanent shaping and curling of the hair, retention of shaping under humidity, and resistance to frizz.