The films of magnesium boride were grown on Si(100) substrate by chemical vapor deposition, starting from the synthesis of the single-source precursor Mg(BH4)(2). The deposition time was varied to study its influence on the film composition. The chemical composition of produced samples was investigated by the XPS technique. The depth profiling of MgBx films was carried out by using Ar ion sputtering combined with cyclic XPS measurements. The contributions of different chemical species (boride and oxides) in the depth profiles were separated by peak-fitting analysis of photoelectron B Is and Auger Mg KLL spectra. In order to determine the correct parameters of peak fitting, the reference samples of MgB2 and MgO were studied as well. The produced samples were composed of MgBx film covered by an overlayer of magnesium and boron oxides, whose thickness was increasing with deposition time. The surface of the films was enriched with magnesium, while the volume was characterized by the B : Mg ratio exceeding 2. XPS results confirmed that the bulk decomposition of Mg(BH4)(2) precursor was successful; therefore, some other phenomenon, such as Mg segregation, was responsible for the formation of MgBx films with x > 2. In addition, a prolonged deposition time resulted in a thicker surface overlayer of Mg and B oxides, but it did not change the stoichiometry of the boride film.