This dissertation focuses on two specific aspects of the chemistry of organosulfur and organoselnium compounds. The first is the internal elimination of the sulfinate esters, sulfonic esters, and the effects of silyl-substitution on the internal elimination of sulfoxides.;The internal elimination of sulfinate esters proceeds through a 5-membered transition state whereas sulfonate esters proceed through a 6-membered transition state. Using computations, it was determined in the 6-membered transition state that substantial charge separation occurs, but less charge separation is observed in the 5-membered transition state. The ability of the sulfonate leaving group to incorporate charge favors the 6-member transition state. In the 5-centered elimination, a nucleophilic-electrophilic mismatch in the transition state is made worse when the sulfur atom is more positively charged, as in the sulfonate.;Silyl groups attached at either the Calpha or Cbeta position of an alkyl sulfoxide lower the activation enthalpy of the sulfoxide syn elimination reaction by a few kcal mol-1. A Silyl substituent at Cbeta can stabilize the transition state by either stabilizing the developing positive charge at Calpha or negative charge at Cbeta. Silyl substitution at Calpha is not positioned to interact favorably with either developing charge in the transition state, and the lower activation barrier is more likely a response to the overall less endothermic reaction than any silyl interaction in the transition state.;The second topic is the photochemistry of organosulfur and organoselenium ylides. The photodeoxygenation of dibenzoselenophene oxide (DBSeO) was found to have a greater quantum efficiency in producing O(3P) than dibenzothiophene oxide (DBTO). However, the photo sensitization and direct irradiation of DBTO (or DBSeO) were found to undergo photodeoxygenation by, in most cases, different mechanisms. Both of these conclusions were supported by monitoring the relative oxidized product ratios. Using calculations, the bond strengths of a variety of sulfur and selenium ylides were determined. This information is used in the elucidation of the photodeoxygenation mechanism of diaryl sulfoxides and selenoxides.