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Results of laser surface alloying (LSA) of 1045 steel with CrB2 and Ni2B powders are presented. LSA was conducted with a 2.5 kW power cw CO2 laser. Laser treated zones were studied by optical and electron microscopy, energy dispersive spectroscopy, and by x-ray diffraction. Formation of complex borides within the treated zones resulted from the dissolution of the injected materials in the melt pool. Additionally, it was established that the dissolution extent determines the hardening mechanism provided by each type of injected material. Dissolution of injected material was found to depend mainly on their melting temperature. A study of the microstructure of treated zones showed that in the case of Ni2B the hardness increase was due to the formation of Fe2B and Fe3B. When CrB2 was used for LSA, an increase in hardness resulted from the dispersion of partially undissolved CrB2 particles, as well as iron borides (Fe2B and Fe3B) formation.