The evaporation of water was monitored from 60, 64, and 68 wt % D2SO4 at 213 K containing 0-0.18 M 1-butanol. Measurements were performed in vacuum using a mass spectrometer to record the velocities and relative fluxes of the desorbing D2O. In addition, the surface activity of butanol in the acid was characterized by hyperthermal argon atom scattering in conjunction with surface tension and butanol evaporation measurements. The segregated butyl species reach surface concentrations of similar to 4 x 10(14) cm(-2) (similar to 80% surface coverage) at 0.18 M bulk concentration. We find that the butyl films do not impede the evaporation of D2O from the acid to within the 5% uncertainty of the measurements. This result implies that small, soluble surfactants such as butanol form porous films that will not alter the growth or shrinkage of supercooled sulfuric acid droplets in the atmosphere.