In previous work the browning reaction in milk was examined by measuring the fall in optical reflectance which occurred during browning. It was then found that the heating of milk caused an initial rise in its reflectance, and, in the present paper this rise in reflectance, or whitening of milk, is examined in more detail. It is shown that the reaction causing it is independent of the subsequent browning. Whitening occurs at temperatures above 60°C., and takes place more rapidly at higher temperatures. The curves of reflectance against time are not exponential, and there is some evidence that the maximum amount of whitening depends on the temperature at which the milk is held. Reduction in pH of the milk causes an increase in the amount of whitening for a particular time-temperature treatment. From these data and other experiments it is concluded that the rise in reflectance is brought about by the denaturation of the soluble proteins of the milk and their subsequent coagulation into particles large enought to reflect light.