Rate constants for the title reaction for different bath gas species, M, are essential to accurate combustion predictions. Most experimental studies of the title reaction assume the reaction is in the low-pressure limit. Furthermore, experimental studies for M = H2O usually rely on measurements in H2O/A mixtures, separate measurements in a reference bath gas M = A, and an assumed mixture "rule," which estimates rate constants in the mixture from those of pure bath gases. We present results from master equation calculations to quantify the uncertainties due to these assumptions in experimental interpretations. Our calculations indicate potential errors due to falloff and mixture rule assumptions that would be imperceptible experimentally over typical variations of pressure and composition yet introduce substantial uncertainties (reaching similar to 50%) in reported rate constants, which often involve extrapolation to zero pressure and unity mole fraction. Going forward, we recommend that experimentally determined pseudo-second-order rate constants for H + O-2 <-> HO2 for the experimental pressure and mixture composition (which are independent of these assumptions) be reported, that experiments used to derive rate constants in the low-pressure limit or for M = H2O be conducted at lower pressures and higher H2O fractions (where these assumptions are more accurate), and that uncertainty analysis consider uncertainties due to falloff and mixture rule assumptions. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.