Abstract: The present invention refers to a new process meant for chrome and/or vegetable leather tanning. The process proposed by the invention comprises the steps of bating or purging, conditioning, and chrome and/or vegetable tannage. The conditioning step uses hydroxy-butaldehyde which reacts by blocking the amines of the collagen and releasing carboyxls. This reaction reduces the isoelectric point to 4.0-4.5, making the hide receptive or susceptible to effective chrome tannage by linking the carboxyls, or to tannin, by linking through hydrogen bridges. Hydroxy-butaldehyde is used in the conditioning step in a proportion of 2.5-3.0% as compared to the weight of the delimed hide, in a concentration of 40.+-.2%, with a pH of 4.2-4.4, and with the reaction time being from 3 to 4 hours.