CASPT2//CASSCF and hybrid DFT calculations have found that the barrier to the concerted ring opening of silacyclopropene (1) to vinylsilylene (2) is lower than the barrier to forming silylacetylene (5). The Delta ZPE-corrected barriers, obtained by (12,12)CASPT2 calculations, are 32.2 and 38.7 kcal/mol, respectively, and the corresponding values obtained by B3LYP calculations are 35.8 and 42.2 kcal/mol. Rearrangement of 1 to silylvinylidene (3) is predicted to be the rate-determining step in the formation of 5. The barrier to ring closure of 2 to 1 is computed to be 31.5 kcal/mol by (12,12)CASPT2 and 31.7 kcal/mol by B3LYP. The CASPT2 barrier height is essentially the same as that for rearrangement of 2 to 1-silaallene (4), but B3LYP predicts that the formation of 4 from 2 requires ca. 3 kcal/mol less than closure of 2 to 1. Our results thus support the hypothesis that the observed generation of equal amounts of the two isotopomers of 1 from monodeuterated 2 is the result of a kinetically competitive equilibrium between 2 and 4. Interconversion between 4 and 5 by two consecutive [1,2]-hydrogen shifts and involving formation of diradical 6 is found to be prevented by high energy barriers (>70 kcal/mol).