Alkaline phosphatase activity, moisture content and bubble volume in spray-dried condensed skim milk were measured experimentally. The activity and moisture content decreased and the bubble volume increased, with increasing outlet-air temperature. Bubble volumes were sometimes larger than the dry solids volume. Scanning electron micrographs contained some particles that were hollow spheres, and others that were caved-in. Mathematical models from the literature which included bubble formation were used to simulate the experiments Predictions generally fell within the range of the experimental results. Activity was predicted to increase with decreasing air temperature and droplet size. In the simulation, most inactivation occurred in a critical region of 70-80% solids, wherein both the moisture content and droplet temperature were sufficiently elevated. Allowance for bubble formation during drying did not significantly affect the predicted moisture content or activity.