The structures and relative rates of formation of the isomeric triplet 1,4-biradical intermediates generated in the photocycloaddition reactions between 2-methylpropene and each of 2-cyclopentenone, 2-methyl-2-cyclopentenone, and 3-methyl-2-cyclopentenone were determined. This was accomplished by using hydrogen selenide as a hydrogen atom donor to trap quantitatively the 1,4-biradicals formed in each reaction, The quantum yields of cycloadduct formation in the photocycloaddition reactions were measured as a function of alkene concentration. For each reaction the relative rates of formation of the biradicals and the quantum yield data were combined to determine quantitatively how each biradical partitions between closure (or disproportionation) to product and fragmentation to ground state enone and alkene. It is concluded that the regiochemistry of the enone-alkene photocycloaddition reactions studied is dominated by the manner in which the biradical intermediates partition between products and ground state precursors and not by the relative rates at which they are formed. It is also concluded that methyl substitution at the 3-position of cyclopentenone has little effect on either the relative rates of formation of the various isomeric biradical intermediates or the manner in which they partition between products and starting materials. However, methyl substitution at the 2-position of cyclopentenone slows formation of biradicals in which the alkene is bonded to the enone 2-position and also inhibits closure to cyclobutane products of biradicals formed by bonding of the alkene to the 3-position of the enone. These results can be rationalized if it is assumed that the enone triplet excited state possesses a planar carbon at the 2-position and a pyramidalized carbon at the 3-position, and if it is also assumed that in the biradicals, radical centers at the cyclopentanone 2-position are planar and at the 3-position are pyramidalized.