The probabilities for N2O5 loss onto sulfuric acid aerosol (60--80 wt % H2SO4, 0.1--0.2 &mgr;m radius, 230--300 K) were measured using a laminar flow reactor coupled to a sulfuric acid aerosol source and a chemical ionization mass spectrometer. The aerosol morphology was extracted from measurements of the aerosol extinction of ultraviolet light (200--370 nm) by using Mie theory. The reaction probabilities, &ggr;, are large (0.06--0.12) and a comparable to probabilities measured previously under similar conditions on bulk sulfuric acid surfaces and small aerosol. These measurements confirm that the overall reaction of N2O5 with H2O in/on 60--80% sulfuric acid is fast and the uptake is due to loss very close to the liquid surface. ¿ American Geophysical Union 1994