Styrene oxide is heated under relatively mild temperature conditions to cause thermolysis. Such thermolysis produces substantial quantities of phenylacetaldehyde in the reaction product. Substantially pure phenylacetaldehyde may be recovered from the crude reaction product by distillation procedure, or other separation means, or, if desired, the phenylacetaldehyde may be directly converted in the crude reaction product to produce its acetals. The process of thermolysis of styrene oxide may be conducted with the styrene oxide in liquid phase, in vapor phase, or in solution within an inert solvent. The conditions of reaction are maintained to achieve a mild decomposition, described as thermolysis, as contrasted to a drastic decomposition, a so-called pyrolysis.