The mass spectra of twelve trisubstituted oyolopropanes have been analyzed and the results may bo summarized as follows:(1) For the cylopropanes with benzoyl,carbonyl or oarbomethogyl substituents,a-cleavage predominated leading to the formation of the [M-C₆H₅CO]⁺,[M-COOH]⁺ and [M-COOCH₃]⁺ ion peaks,so their moleoular ion peak were very small.(2) For traps-1,2-oyolopropane dioarboaylio a,oid the notable peaks were [M-H₂0]⁺ and [M-H₂O-CO]⁺,but for the ois isomer it was [M-H₂O]⁺.