Primary cations of the form XCH2CH2+ (1) are unstable with respect to the bridged isomer cyclo-(CH2)(2)X+ (2) or the hydride-shift isomer CH3CHX+ (3). Two independent hydrogen isotope experiments for X = OH demonstrate that gaseous HOCH2CH2+ (Pal gives less of its bridged isomer, protonated oxirane (2a), than of its hydride-shift isomer, protonated acetaldehyde (3a).Metastable ion decompositions of ionized HOCH2CH2OPh show that the radical cation decomposes via an ion-neutral complex containing 3a and phenoxy radical, Deuterium labeling studies exhibit no detectable interconversion of the two sp(3)-carbons, implying that complexes containing 2a are not formed to a measurable extent. An independent neutral product study looks at the radioactive oxirane and acetaldehyde produced when tritium on the methyl group of gaseous CH2TCHTOH undergoes radioactive decay. Loss of a beta-particle and a helium atom forms transient, tritiated la. Ions from isomerization of this radiolabeled primary cation were deprotonated by Me3N and gave a >97% yield of acetaldehyde. Quantitative assessment of possible routes to acetaldehyde (including rearrangement of excited 2a) implies that 1 undergoes hydride shift at least 5 times faster than bridging by neighboring oxygen.