The self-reaction of the HO2 radical is an important termination reaction for HOx and it is the major gas phase source of hydrogen peroxide in the atmosphere. It is well established that the rate of this reaction is sensitive to both pressure and water vapor concentrations. The NASA recommended rate parameter (DeMore et al., 1992) has been utilized in several detailed gas phase chemical mechanisms for modeling the troposphere, but several other recent studies have omitted one or both of the terms for the rate constant. If both the pressure and the water dependent terms are omitted the relative error can be as high as 75% at the surface to near 30% at 10 km for water vapor saturated conditions. Models which omit these terms will predict incorrect HOx budgets, underestimate gas phase hydrogen peroxide formation rates, and overestimate ozone and organic peroxide formation rates especially in clouds or under marine conditions. ¿ American Geophysical Union 1995