IR absorption spectra of monoisotopic silane (28)SiH4 and germane (76)GeH4 are studied in nitrogen matrixes at T = 11 K. It is shown that the absorption spectra of silane and germane are similar in the regions of the ν3 stretching and ν4 bending vibrations. A significant influence of a nitrogen matrix on the absorption spectra of XH4 molecules was established. The bands in the ν3 region of silane and germane in a nitrogen matrix are blue-shifted. The spectra of monomer molecules of SiH4 and GeH4 contain a triplet in the stretching region and a doublet in the bending region, which is explained by the change in the molecular symmetry of an XH4 molecule from T(d) to C3 on passage from the gas phase to solid nitrogen. The calculations of the matrix structure were made by the Monte Carlo method with the ensemble of particles containing the SiH4 molecule and 863 N2 molecules. The calculations predicted a set of equilibrium structures with the C3 symmetry. The structure of the absorption bands of the dimer is analyzed in the ν3 stretching and ν4 bending regions. The intensity of these absorption bands increases quadratically with concentration. The contribution of resonance dipole-dipole interactions in the formation of the absorption bands of (XH4)2 dimers in the ν3 stretching and ν4 bending regions is discussed.