The ER-2 made two descents through upper tropospheric biomass burning plumes during ASHOE/MAESA. HOx(=OH+HO2) concentrations are largely self-limited outside the plumes, but become progressively more limited by reactions with NOx(=NO+NO2) at the higher NOx concentrations inside the plumes. Sources of HOx in addition to H2O and CH4 oxidation are required to balance the known HOx sinks both in the plumes and in the background upper troposphere. HOx concentrations were consistently underestimated by a model constrained by observed NOx concentrations. The size of the model underestimate is reduced when acetone photolysis is included. Models which do not include the additional HOx sources required to balance the HOx budget are likely to underestimate ozone production rates. ¿ 1997 American Geophysical Union