Raw skim milk, with or without added CO 2 , was heated, held, and cooled in a small pilot-scale tubular heat exchanger (372ml/min). The experiment was replicated twice, and, for each replication, milk was first carbonated at 0 to 1°C to contain 0 (control), 600, 1200, 1800, and 2400ppm added CO 2 using a continuous carbonation unit. After storage at 0 to 1°C, portions of milk at each CO 2 concentration were heated to 40, 56, 72, and 80°C, held at the desired temperature for 30s (except 80°C, holding 20s) and cooled to 0 to 1°C. At each temperature, five pressures were applied: 69, 138, 207, 276, and 345 kPa. Pressure was controlled with a needle valve at the heat exchanger exit. Both the pressure gauge and pH probe were inline at the end of the holding section. Milk pH during heating depended on CO 2 concentration, temperature, and pressure. During heating of milk without added CO 2 , pH decreased linearly as a function of increasing temperature but was independent of pressure. In general, the pH of milk with added CO 2 decreased with increasing CO 2 concentration and pressure. For milk with added CO 2 , at a fixed CO 2 concentration, the effect of pressure on pH decrease was greater at a higher temperature. At a fixed temperature, the effect of pressure on pH decrease was greater for milk with a higher CO 2 concentration. Thermal death of bacteria during pasteurization of milk without added CO 2 is probably due not only to temperature but also to the decrease in pH that occurs during the process. Increasing milk CO 2 concentration and pressure decreases the milk pH even further during heating and may further enhance the microbial killing power of pasteurization.