The effect of flow-induced relative humidity changes on cut sizes of aqueous sulfuric acid particles in a multi-nozzle microrifice uniform deposit impactor (MOUDI) was studied theoretically and experimentally. In the theoretical study, the differential equation of vapor transport to particles including the Fuchs correction, the Kelvin effect, sulfuric acid solution properties, and temperature depression was integrated along the particle critical trajectory to determine the cut size at various relative humidities. Results show that aqueous sulfuric acid particles first shrink owing to evaporation caused by pressure drop, then grow in the jet owing to condensation caused by aerodynamic cooling. The study was confined to stages 7 (50% cut size = 0.122 μm) and 8 (50% cut size = 0.047 μm) where humidity variations are greatest. It was found that the net size change of aqueous sulfuric acid droplets is negligible when the inlet relative humidity is < 80% because of the moderate pressure drop across nozzles and the extremely short residence time the particles spend in the jet. The agreement between theory and experiment is good.