The adsorption and thermal decomposition of N2H4 on Si(100)-2 × 1 were investigated using XPS, UPS, HREELS and LEED. Upon adsorption of 0.2 L N2H4 on the surface at 100 K, partial dissociation of the NH bonds was indicated by the appearance of the Si-H stretching mode at 255 meV in HREELS and the 399.0 eV peak in N 1s XPS, which is assigned to the N2Hx (presumably N2H3) species. At lower dosages (D < 0.4 L), N2H4 adsorbed on Si(100)-2 × 1 in a side-on rather than an end-on geometry. At higher dosages, hydrogen-bonding is noted to be formed in the overlayer adsorbates. The NN bond breaking process was observed to occur at 600 K; the NHx products (x = 0, 1, 2) could be identified on the surface. At this temperature, the NN stretching mode essentially disappeared and the NH2 deformation and the NH stretching modes shifted from 200 and 413 to 192 and 420 meV, respectively. The same conclusion could also be established from the corresponding XPS and UPS results. Annealing the surface to higher temperatures caused further NH bond breaking, accompanied by the desorption of H2, while N remained on the surface forming Si nitride.