The photochemistry of substituted cinnamyloxy silanes has been examined in both cyclohexane andacetonitrile solvents. Alkene isomerization occurs in addition to cycloaddition. Fluorescence quantumyields and excited singlet state lifetimes have been determined for each compound. We have used theinformation in order to better understand the regio- and stereoselectivity of photocycloaddition betweensilyl-tethered cinnamyl groups. This study allows us to conclude that the 2 + 2 photocycloaddition betweenalkenes is not a Woodward−Hoffmann orbital symmetry controlled event. The most consistent explanationfor the excellent regio- and stereoselectivity is that the photocycloaddition is conformationally controlled.