The regiochemistry of the photochemical cycloaddition reaction between 2-cyclopentenone and the polar alkenes 1,1-difluoroethene and methyl acrylate has been investigated. With 1,1-difluoroethene the major cycloaddition product is a cyclobutane adduct possessing head-to-tail regiochemistry; with methyl acrylate the reaction is non-regioselective and a 46.5:53.5 ratio of head-to-head and head-to-tail cyclobutane regioisomers is obtained. These regiochemical outcomes are not those predicted by the Corey-de Mayo mechanism conventionally used to describe the enone-alkene photocycloaddition reaction. The triplet 1,4-biradicals implicated as intermediates in the photocycloaddition reaction between 2-cyclopentenone and the polar alkenes methyl acrylate and ethyl vinyl ether have been trapped quantitatively using hydrogen selenide as a hydrogen atom donor. The structures of the trapped products have been determined and hence the structures of their biradical precursors have been inferred. The yields of the trapped biradicals indicate that in both reactions the product distribution is controlled by the extent to which each of the isomeric biradical intermediates closes to products in competition with fragmentation to its ground state precursors. The product distribution does not reflect the relative rates of formation (and hence the relative amounts) of the isomeric biradical intermediates formed in each reaction. This conclusion is inconsistent with the long held idea that an oriented exciplex intermediate controls the reaction regiochemistry by favoring the formation of some biradicals over others. The structures of the biradical intermediates also reveal that both the 2-position and the 3-position of the enone are involved in forming the first bond to the alkene; with ethyl vinyl ether this bond is to the less substituted end of the alkene exclusively, while with methyl acrylate no selectivity in the site of initial bonding is observed.