IR absorption spectra of monoisotopic silane (SiH4)-Si-28 and germane (GeH4)-Ge-76 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 nu(3) stretching and nu(4) bending vibrations. A significant influence of a nitrogen matrix on the absorption spectra of XH4 molecules was established. The bands in the nu(3) 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 Td to C-3 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 N-2 molecules. The calculations predicted a set of equilibrium structures with the C-3 symmetry. The structure of the absorption bands of the dimer is analyzed in the nu(3) stretching and nu(4) 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 nu(3) stretching and nu(4) bending regions is discussed.