The stopping cross sections of acetaldehyde, ethylene oxide, propylene oxide, and vinyl methyl ether for 0.3–2.0-MeV accelerated He+ions have been measured with probable random errors of 0.5–0.9&percent;. The stopping cross sections of the three-membered ring-structure compounds ethylene oxide and propylene oxide were found to be lower than those of their double-bonded isomers, but higher than those of isomers calculated from single-bonded alcohols.