There has been considerable debate and speculation about the role of weakly bound complexes in radical molecule reactions in the gas phase, especially in atmospheric chemistry. Among the significant number of potentially important molecular aggregates in chemical reactions, water complexes are of particular interest. Beyond the well-known energy transfer role of water in complex-forming reactions, it has been shown that water may also have a catalytic effect on the kinetics of radical-molecule reactions because of reduced reaction barrier heights for the complexes. Here we report studies of the reaction of OH radicals and propionaldehyde in the presence and absence of water vapor between 300 and 60 K in Laval nozzle expansions. Water accelerates the overall reaction at low temperatures but much less pronounced than for the reaction of OH with acetaldehyde reported recently. Quantum chemical calculations help us to understand this behavior, which can be rationalized in terms of the stability of intermediate reaction complexes and the effect of water aggregation on the barrier separating prereactive complexes and products.