The present invention provides a hair straightening composition with improved ease of use and reduced smoke development during the application of a straightening iron. The composition comprises at least one carboxylic acid of the formula (I) in combination with at least one polymeric thickening agent selected from non-ionic, anionic, cationic and amphoteric polymers having a viscosity of at least 500 mPa-s measured at a polymer concentration of 1% by weight, in water and at 20° C. with a Brookfield viscometer: R—CO—COOH Formula (I) In further aspects, the present invention concerns a process for semi-permanent straightening of the hair, utilizing said composition, the use of the composition for straightening hair, and a kit comprising the composition and a straightening iron.