The pH and calcium activity of reconstituted skim milk solutions (9-21 w/w milk solids nonfat) on heating and after cooling were studied as a function of milk pH prior to heating (pH 6.2-7.2 at 25 degrees C) and added calcium complexing agents (phosphate or EDTA). The pH decreased as the temperature was raised from 25 to 90 degrees C and the magnitude of the pH decrease was greater with increase in initial pH at 25 degrees C before heating or milk concentration. The pH decrease on heating from 25 to 90 degrees C in skim milk solutions with added calcium complexing agents was lower than that of milk without the addition of these salts. The calcium activity decreased on heating from 25 to 60 degrees C. The magnitude of the change decreased with increase in initial pH at 25 degrees C before heating and milk concentration. The decrease in calcium activity on heating from 25 to 60 degrees C for skim milk solutions with added calcium complexing agents was lower than that of milk solutions without the addition of calcium complexing agents. The changes in pH and calcium activity on heating milk were largely reversible after cooling the milk. The results suggested that the pH and calcium activity at high temperatures are a function of the milk composition. Knowledge of the initial pH prior to heating alone is not sufficient for predicting the changes that occur during heating.