The presence of Lewis bases causes the zero-phonon Q band of matrix-isolated dihydrogen to become more intense and to shift to lower frequency than is observed in hydrogen-doped argon matrices. Because the intensity of the absorption due to the perturbed H-H stretch is not strongly a function of the H2 concentration in the matrix, it is reasonable to presume that the intensity arises from a H2⋯base pair. Bases that have been shown to affect the spectrum of H2 in this manner include water, acetone, ammonia, pyridine, trimethylamine, and trimethylphosphine. The magnitude of the frequency shifts suggests that these associations should be considered van der Waals molecules. As is the case for solid hydrogen, the observation of the H-H stretch by infrared methods depends on the rotation of the hydrogen in the van der Waals complex, although Raman measurements suggest that nonrotating hydrogen also forms a complex. Because of slow nuclear-spin relaxation, a large proportion of H2 and D2 survive at 10 K in the J = 1 rotational state. Thus, the H-H and D-D stretch, but not the H-D stretch, of complexed hydrogen can be easily observed in the infrared spectrum. The H-D stretch becomes observable, however, at higher H-D concentrations in the presence of adventitious H2.