Rate coefficients and product branching ratios for the ion-molecule reactions of the hydroxyl cation, ionized water, and their deuterated analogues with ethylene have been determined using a selected ion flow tube (SIFT) at room temperature and in 0.5 Torr of helium buffer gas. In all cases, reactions proceed at or near the collision rate, The major product is always charge transfer: 79% for L2O.+ and 66% for LO+ and does not depend on the isotopic form of hydrogen present (L = H or D). For the L2O.+ reactions, the remaining 21% of products are from proton or deuteron transfer, with no evidence of an isotope effect on this step even in the HOD.+ reaction. The greater exothermicity of the initial charge transfer in the LO+ reaction is revealed by the observation of additional product channels, forming the vinyl cation and protonated carbon monoxide. Multistep mechanisms that proceed through rate-determining charge-transfer, followed by a product-determining step, are postulated to explain these observations.