The decomposition of diborane (B2H6) has been studied on the Ni(100) surface by means of thermal desorption spectrometry (TDS), x-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At low gas exposures, decomposed B2H6 on the Ni(100) surface at low temperatures produces several peaks in the H-2 TDS spectra. At high gas exposures, B4H10 was observed desorbing from the surface. The TDS spectra and XPS results for low gas exposures indicate that heating to 500 K results in complete decomposition of B2H6, leaving B bonded to the Ni(100) surface. Heating a thick boron film to 700 K results in a significant shift in the B [1s] and Ni [2p(3/2)] XPS peak positions, indicating that adsorbed B reacts with Ni(100) to form a nickel boride phase. This nickel boride phase is stable on the surface at temperatures up to 900 K. Nickel boride standards (NiB, Ni2B, Ni3B) were used for comparison with the surface nickel boride phase. XPS analysis of the standards led to the determination that the phase forming on the surface at 700 K was Ni2B. As the temperature is increased above 900 K, XPS results indicate that the boron diffuses rapidly into the bulk. It has also been found that high-temperature annealing in the presence of B2H6 results in the formation of a sharp (3x3) LEED pattern produced by 2/9 ML of B on the Ni(100) surface.