Results of this study confirm that high temperature (118°C/15 min) and high pressure (400 MPa/5 min) processing of skim milk, skim milk ultrafiltration and ultracentrifugation fractions, and model milk salt solutions cause dramatic shifts in their colloidal and soluble Ca phospate equilibrium that affect their pH, dissolved Ca content, turbidity, and casein micelle microstructure. The relations between high temperature and high pressure processing-induced changes in the colloidal and soluble Ca phosphate equilibrium were evaluated in raw, pasteurized, and high temperature treated skim milk, ultrafiltration retentate and permeate of pasteurized skim milk, clear ultracentrifugation infranatant of pasteurized skim milk, and synthetic milk ultrafiltrates with and without lactose or Ca. The magnitude of the pH and dissolved Ca shifts caused by high temperature and high pressure processing was a function of casein micelle concentration. Ultrafiltration permeate exhibited the most drastic shifts in pH and dissolved Ca contents due to high temperature and high pressure processing. Although high temperature processing reduced the pH of ultrafiltration permeate from 6.59 to 6.03 and the dissolved Ca from 100% to 58%, high pressure processing reversed both of these changes. These changes in high temperature and high pressure processed milk, milk fractions, and model milk salt solutions were related to microstructural changes in the casein micelles as revealed by electron microscopy.