The UV photolysis of the α, β-unsaturated aldehydes acrolein, methacrolein and crotonaldehyde in argon matrices at λ> 300 nm has shown a major photochemical deactivation pathway to be conformational isomerism to the thermodynamically less stable s- cis form. This type of isomerization may account for the rapid internal conversion observed in the gas phase. Matrix-isolated crotonaldehyde undergoes further isomerization to ...