We studied the inactivation of trypsin and alpha-and beta-chymotrypsin by passage of droplets of tridecane though their aqueous solutions. The mechanism involves contact with the interface, because the toss of activity is proportional to the total area exposed. The rates of inactivation vary up to fivefold over the pH range 3 to 10. However, there is no clear maximum at the isoelectric point (pl) of each enzyme, where the amount of protein adsorbed is usually found to be highest. This is probably because, at the pl, there is also a minimum in structural alteration on adsorption. There may be a weak correlation with pH effects on foamability of the enzyme solutions, a parameter reported to reflect the "hardness" of different proteins, which controls their interfacial unfolding. The pH dependence of both inactivation and hardness cautions against attempts to correlate inactivation of different enzymes with a single value of a parameter such as adiabatic compressibility. There is no correlation between the effects of pH on interfacial inactivation and those reported in the literature on irreversible inactivation in concentrated urea or at high temperature. (C) 2000 John Wiley & Sons, Inc.