At an average temperature of 6 °C., exposure of both whole and skim milk powders to sunlight caused more rapid deterioration in quality than occurred in the dark. Ultra-violet light with a principal wave length of 3800 Å accelerated deterioration in whole milk powder stored at 38 °C., but had no significant effect on skim milk powders; the effect of this light on whole milk powders was less pronounced than that produced by sunlight. Storage of samples at 38 °C. under different light intensities indicated that the differences between ultraviolet and sunlight were the result of the difference in total energy of light falling upon the sample, rather than the difference in wave length of the activating light.