For the first time [3 + 2] 1,3-cycloaddition of an ionized carbonyl ylide has been observed in gas phase ion-molecule reactions of (+CH2OCH2.) (1) with several carbonyl compounds. The reaction, which competes with electrophilic addition that leads to net CH2.+ transfer, occurs across the C=O double bond of acetaldehyde and several acyclic ketones yielding ionized 4,3-dialkyl-1,3-dioxolanes as unstable cycloadducts. Rapid dissociation of the nascent cycloadducts by loss of a 4-alkyl substituent as a radical leads to the observed products, that is cyclic 3-alkyl-1,3-dioxolanylium ions. Cycloaddition of 1 with cyclic ketones yields bicyclic spiro adducts, which also undergo rapid dissociation. Cyclobutanone yields ionized 1,3-dioxaspiro[4,3]octane, which dissociates exclusively by neutral ethene loss to ionized 4-methylene-1,3-dioxolane. Ionized 1,3-dioxaspiro[4,4]nonane is formed in reactions with cyclopentanone, and its rapid dissociation by loss of C3H6 and C2H5. yields the ionized 4-methylene-1,3-dioxolanylium and the 4-ethenyl-1,3-dioxolanylium product ions, respectively. A systematic study of this novel reaction and characterization of the product ions carried out via pentaquadrupole (QqQqQ) multiple stage (MS-(1) and MS3) mass spectrometric experiments provide experimental evidence for the cycloaddition mechanism. The dissociation chemistry observed for the cycloaddition products correlate well with their proposed structures and was compared to that of both isomeric and reference ions. Ab initio MP2/6-31G(d,p)//HF/6-31G(d,p) + ZPE potential energy surface diagrams for the reactions of 1 with acetone, fluoroacetone, and 1,1,1-trifluoroncetone support the operation of the two competitive reaction pathways, that is CH2.+ transfer and [3 + 2] 1,3-cyclonddition/dissociation, and show that the cycloaddition process is favored by electron-withdrawing substituents.