The effect of high pressure thermal processing (HPTP), in the range of 0.1-600 MPa and 100-140 degrees C for up to 60 min, on colour change and proteolysis of reconstituted skim milk (10% w/w) was investigated. The kinetic results showed that colour change (Delta E-ab) and proteolysis (determined by chromatography) increased with both increasing temperature and pressure. The apparent reduction of free amino groups in skim milk, indicating sugar conjugation to milk proteins/peptides, was accelerated with increasing temperature, but decelerated with increasing pressure (at constant temperature) at higher temperature. The milk's colour changed drastically at 400 MPa where most of the milk proteins formed coagulates and left the solutions nearly translucent. Mathematical models describing the kinetics of colour change, proteolysis, and free amino acids reduction as a function of pressure and temperature are proposed. Crown Copyright (C) 2014 Published by Elsevier Ltd. All rights reserved.