Accurate rate coefficients for 35 1,2-hydrogen shift reactions for hydrides containing up to 10 silicon atoms have been calculated using G3//B3LYP. The overall reactions exhibit two distinct barriers. Overcoming the first barrier results in the formation of a hydrogen-bridged intermediate species from a substituted silylene and is characterized by a low activation energy. Passing over the second barrier converts this stable intermediate into the double-bonded silene. Values for the single event Arrhenius pre-exponential factor, (A) over tilde, and the activation energy, E-a, were calculated from the G3//B3LYP rate coefficients, and a group additivity scheme was developed to predict (A) over tilde and E-a. The values predicted by group additivity are more accurate than structure/reactivity relationships currently used in the literature, which rely oil a representative (A) over tilde value and the Evans-Polanyi correlation to predict E-a. The structural factors that have the most pronounced effect on (A) over tilde and E-a were considered, and the presence of rings was shown to influence these values strongly.