The amount of SO2 gas absorbed by ice of known surface area at equilibrium was used to estimate the volume of liquid water present at the ice-air interface at temperatures from -1 to -60 ¿C. Calculations were based on Henry's law and acid dissociation equilibrium. The liquid volume is lowest at lower temperatures and ionic strength and under most conditions was greater than the volumes calculated based on freezing-point depression. The equivalent liquid layer thickness, assuming that liquid water is uniformly distributed around the grains, ranged from 3--30 nm at -60 ¿C to 500--3000 nm at -1 ¿C. Corresponding ionic strengths for the two temperatures were 1.7--0.0012 M and 0.005--0.00009 M. Lower values were for ice made from distilled water, and higher values were for ice made from 10-3 M NaCl. Estimated pH values were from 2.9 at -60 ¿C to 4.1 at -1 ¿C. Results demonstrate that gas absorption can be used to estimate an equivalent liquid volume and thickness for the ice-air interfacial region. While not directly comparable to physical measurements, the estimated values should be directly applicable to modeling uptake of SO2 and other trace gases by ice. Lack of good thermodynamic data for temperatures below 0 ¿C is the main limitation to applying this method. ¿ American Geophysical Union 1993