Laboratory data for reactions of ClONO2 or HOCl with HCl on nitric acid trihydrate and sulfuric acid tetrahydrate are compared using a surface reaction model. Data sets where both non-reactive and reactive uptake were measured reveal that changes in the reactive uptake coefficients of ClONO2 and HOCl can be explained completely by the amount of HCl taken up by the hydrates. Large discrepancies are revealed between measurements from different laboratories which have important consequences for calculated ozone destruction (see companion paper II).¿ 1997 American Geophysical Union