Low-temperature irradiations of 1-phenylcyclohexene (PC6) yieldtwo singlet state 2 + 2 photodimersand two 4 + 2 cycloadducts. The 4 + 2 adducts are not observedin low intensity lamp irradiations at room temperaturebut are produced in significant quantities when PC6 solutions aresubjected to higher intensity laser irradiation. Theresults indicate a reaction mechanism involving twotrans-PC6 molecules. The findings are consistent withearlierkinetic observations that the decay of trans-PC6 adopts asignificant second-order component at lower temperatures.The transient-transient reaction also occurs undertriplet-sensitized low-temperature/low-intensity or roomtemperature/high-intensity irradiation conditions. Triplet-sensitizedirradiations also yield significant amounts of the 2 + 2photodimers produced by attack of the PC6 triplet on its ground state.Quantum yield studies reveal that the rate ofaddition of 3PC6* to its ground state is in the range0.28 to 4.0 × 105 L mol-1 s-1consistent with the 1,2-biradicalmodel for alkene triplet reactivity.