Investigation of the effect of pH on the formation of deposit from milk during ultra high temperature treatment using a plate-type plant showed that deposit formation was greatly increased when the pH of whole milk was reduced to 6-54, irrespective of whether the adjustment was made through the addition of HC1 or lactic acid. Most of the increase in deposition took place in the higher temperature sections of the plant. Conversely, an increase in milk pH to 6-8 using NaOH resulted in considerably less deposit being formed during heat treatment. Reducing the pH of whole milk increased the deposition of both protein and fat, but reduced the deposition of minerals. Despite very high concentration of fat in the deposits, it is unlikely that fat per se was responsible for increased deposit formation. Deposition also increased when the pH of skim milk was reduced to 6-51 before processing. Electron micrographs of the milks after heat treatment indicated that pH reductions caused the formation of large aggregates containing casein micelles during heating. Fat globules were also present in aggregates formed in whole milk with reduced pH. Slight reductions in the pH of milk before processing appear to enable the pH during heat treatment to fall below a critical value at which coagulation of milk takes place at the heated surfaces.The deposition of milk solids in heat treatment plant is a major problem in the dairy industry. Deposit formation during ultra high temperature (UHT) processing using tubular or plate-type plant results in the build-up of high pressures in the heat exchangers and thermal efficiency is reduced (Burton, 1968;Swartzel, 1983). Within a relatively short time a limiting pressure, as determined by the resistance of the plate gaskets to internal pressure, is reached. Processing must be stopped and the deposit removed by chemical cleaning before heat treatment of milk can be continued.The composition of deposits formed from milk during UHT processing is dependent on the temperature of the heated surface and on the heat treatment previously given to the milk (Ito et al., 1962a, b; Lyster, 1965;Burton, 1966;Skudder et al. 1981;Lalande et al. 1984). Most deposit normally forms in sections of the plant operating at 100-105 °C and consists of protein (50-60%), minerals (30-35%) and fat (15-20%). At higher temperatures the concentration of minerals