The purpose of this study was to investigate the heat stability and calcium ion activity of the reconstituted MPC dispersions. Reduced calcium MPC (RCMPC) powders were produced from skim milk subjected to CO2 treatment before and during the process of ultrafiltration. The CO2 injection was controlled to obtain 0, 20, 30, and 40% reduction in Ca levels of MPC powders. RCMPC powders were reconstituted to 10% protein in deionized water. These dispersions were tested for heat stability in a rocking oil bath at 140°C at unadjusted, 6.5, 6.7, 6.9, and 7.1 pH. Calcium ion activity (CIA) and ionic strength measurements on Ca-standards and all dispersions were carried out using a Ca ion-selective electrode and conductivity meter. The unadjusted pH of the dispersions varied from 6.8 in control to 5.95 in 40% RCMPC dispersions. This pH difference had an effect on the CIA of the dispersions and ranged from 1.31mM in control to 18.39mM in 40% RCMPC dispersions. Differences in the heat stability were observed in the unadjusted pH dispersions with the control MPC having the highest heat stability that decreased as the percentage of Ca reduction increased. The heat stability of control MPC dispersions showed a minimum at pH 6.7 and maximum at pH 6.9 with a decrease observed at pH 7.1. The CIA of RCMPC dispersions decreased with an increase in pH. A steady increase in the heat stability of 30 and 40% RCMPC dispersions was observed with increase in pH which correlated with decrease in the calcium ion activity. Dispersions from 20% RCMPC showed a heat stability profile similar to the control MPC dispersion but had higher heat stability values at all adjusted pH values. The CIA activity had a direct correlation on the heat stability of the unadjusted MPC dispersions, but at a higher pH of 7.1, no correlation was evident, even though, all the dispersions had a low CIA. Heat stability of 30 and 40% RCMPC dispersions increased with an increase in the pH showing highest heat stability at a pH of 7.1. From this study, it can be concluded that pH and ca-ion activity influences the heat stability of RCMPC dispersions.