The photolysis of either cis- or trans-but-2-ene produces hydrogen, thirty-two hydrocarbon products and a solid polymer. The differing effects of oxygen, nitric oxide and a number of added inert gases on the product yields, enabled the free radical products to be distinguished from the molecular fragments. A mechanism is proposed in which eight possible primary reactions account for the products. All the primary reactions except that which forms the other isomer, may be quenched by increasing the pressure in the system or adding foreign gases, but in the quenching process only the original isomer is formed; there is no change in the amount of isomerization. To explain this result, it is suggested that the initial absorption produces the singlet Rydberg excited state, which may decompose or be deactivated to the original isomer. The isomerization which does occur is not affected by pressure or by the addition of reactive gases, and is thought to occur through the V(π*, π) state.