The present invention relates to a process for semi-permanent straightening and dyeing hair for improved and milder semi-permanent straightening and dyeing of hair, especially human hair. The inventors have unexpectedly found that when commonly used oxidative dyeing compositions are mixed with another composition comprising predominantly carboxylic acids, the dyeing effect of the composition is improved, homogeneous dyeing of hair fibers is achieved and natural cosmetic properties of hair is maintained. The hair may be semi-permanently straightened before or after oxidative dyeing the hair.