The recently revived interest in the borate treatment of hem-fir has lead to the search for more rapid treatment processes. Pressure treatment combined with a short diffusion period shows promise, particularly for Pacific silver fir, but further improvements were needed to rapidly achieve penetration of the full cross section in western hemlock. Didecyldimethyl ammonium chloride (DDAC) had been added as a mould inhibitor during the diffusion process but it might also have an effect on uptake through action as a surfactant. End-matched specimens of 105 x 105 mm cut from hemlock baby squares were treated with Tim-Bor to which various concentrations of DDAC were added. Addition of 0.5% DDAC substantially increased the penetration of borate into hem-fir during pressure treatment. Concentrations as low as 0.05% DDAC improved penetration during storage after treatment. A second experiment confirmed the penetration enhancement of higher concentrations during pressure treatment and of lower concentrations during subsequent storage.