Isomers of bicyclol3,2,01heptan-2-one trimethylsiloxy derivatives, produced by ( n 2 + . 2 ) photocycloaddition reactions between cyclopent-2-enones and trimethylsiloxy alkenes, have been investigated by methane and isobutane chemical ionization mass spectrometry. Head-to-head and head-to-tail regioisomers and the syn-and anti-epimers in each regioisomeric set can be identified by means of these ionization methods. Prominent fragmentation patterns are discussed.