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H 2 is a fascinating molecule whose properties revealed the influence of nuclear spin on the molecular wave function in the 1920s. As an interstitial defect in Si, the H 2 molecule has given rise to a number of perplexing puzzles since the discovery of its vibrational spectrum. The absence of an ortho-para splitting for the H 2 vibrational line and an apparent low symmetry found in stress experiments misled several researchers into thinking that interstitial H 2 in Si must have a barrier to rotation. Our discovery of a new vibrational line for HD in Si and its interpretation, along with the recognition that certain transitions are possible for HD, but not for H 2 or D 2 , establish that H 2 in Si is a nearly free rotator after all. Additional puzzles such as the anomalous intensity of the HD line, the absence of an isotope dependence for the uniaxial stress splitting of the H 2 and D 2 vibrational lines, and the properties of an O-H 2 complex are also explained naturally. Recent Raman studies confirm that interstitial H 2 in Si is a free rotator but raise interesting new questions about the diffusivities of the ortho and para species.