The paper presents study results of laser alloying of CT90 tool steel with an applied pre-coat of boron, molybdenum or a mixture of these elements. Pre-coats were applied on steel substrates in the form of a paste. The aim of the study was to investigate the microstructure, chemical and phase composition, microhardness and corrosion resistance of these newly-formed coatings. The laser alloying process was carried out using a diode laser with a nominal power of 3 kW. In this study a laser beam power of 900 W and a scanning speed of 48 mm/s were used. As a result of the laser beam action, the presence of three areas was observed in cross-sections of specimens: a remelted zone, a heat affected zone and the substrate. The properties of coatings enriched with both molybdenum and boron were better than those of the steel substrate, but only the use of a Mo-B mixture resulted in a significant improvement in microhardness and corrosion resistance.