The uptake coefficient for the loss of peroxyacetyl radical, CH3C(O)O2 (PA), on H2O is measured to be 0.0043 (+0.0024/-0.0015) at 274¿3 K. Uptake measurements onto aqueous solutions of sodium ascorbate yield limits for the PA Henry's law coefficient in H2O (≤0.1 M atm-1) and the CH3C(O)O2 hydrolysis rate coefficient (≥7¿105 M-1 s-1) at ~275 K. These measurements support the hypothesis that PA heterogeneous reaction can mediate the loss of PAN in fog (Roberts et al., submitted manuscript, J. Geophys. Res., 1995). The most likely products of PA hydrolysis are HO2 and acetic acid. PA hydrolysis may be a significant source of odd hydrogen at night as well as an important source of acetic acid in the atmosphere. ¿ American Geophysical Union 1996