IR absorption spectra of monoisotopic ²⁸SiH ₄ and ⁷⁶GeH ₄ are studied in Ar and N ₂ matrices at 10 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the stretching v ₃ and bending v ₄ vibrations. Four groups of bands can be separated out in the spectra of each molecule: (1) narrow bands characteristic of the matrix isolation studies, (2) broad bands, (3) diffuse absorption with a large value of the spectral moment M* ₂ the intensity of which increases upon annealing, and (4) bands of dimers the intensity of which increases quadratically with concentration. The spectra of ²⁸SiH ₄ and ⁷⁶GeH ₄ in nitrogen matrices contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry from T _d to C _3V on passage from the gas phase to solid nitrogen.