A new method is presented for the enlargement of particle size through condensation of water vapor in a laminar, thermally diffusive flow. The method involves the introduction of an air flow at temperature Ti into a wet-walled tube at a temperature Tw > Ti. This approach yields higher supersaturation values than either mixing or cold-walled condensers when operating between the same temperature extremes. Model results for the saturation profiles within the condensing region show that the peak supersaturations are reached along the centerline of the flow, and that the activation efficiency curves are steeper for large temperature differences when the cutpoint diameter is smaller. Experiments conducted with three types of aerosol, oleic acid (a water-insoluble oil), a mixture of oxalic acid and sulfate, and with ambient laboratory aerosol confirmed that condensational growth is achieved with this approach, although experimental cutpoints are somewhat higher than predicted for wettable particles.