A new apparatus is described that allows measurement of the effects of rapid surface expansion, due to a drop in pressure, on the stability of air bubbles beneath a planar air-water (A-W) interface. The fraction, F-c, of bubbles coalescing on expansion has been studied as a function of bulk protein concentration, expansion rate, and surface age of the interface before expansion. Two commercial protein samples were used: a whey protein isolate (WPI) and a sodium caseinate (SC). For 0.04 wt% WPI, when bubbles were slowly compressed and then rapidly expanded back to their original area, F-c was found to increase with decreasing bulk protein concentration (C-b) and increasing bubble size, but all values of F-c were relatively low, that is, less than 0.1, under conditions where bubbles were reasonably stable on injection. Much higher values of F-c were observed when bubbles were rapidly expanded from their original size, that is, with no prior compression stage. Here F, was also observed to increase with decreasing bulk protein concentration and increasing expansion rate. Within experimental error, F-c was not very dependent on the age of the planar A-W interface, except for 0.2% WPI, when there was a slight increase in F-c with increasing surface age. In contrast, once stable bubbles had been formed at the A-W interface with SC as the stabilizer, they remained very stable to coalescence, that is, with F-c approximate to 0, for all bulk concentrations, expansion rates, and surface ages studied. The results can be explained mainly in terms of the adsorption kinetics and surface rheology of the two proteins at the A-W interface, although a number of observations suggest that other phenomena, such as the variable tendency for the bubbles to move and to aggregate in the interface, also have an influence on bubble stability.