Skim milk fortified with all-trans retinyl palmitate, using corn or coconut oil as the vitamin carrier, was studied to determine the effect of physical state and droplet size of the carrier system on loss of all-trans retinyl palmitate during exposure to light. To determine effect of physical state, fortified samples were exposed to light at 4 degrees C, where corn oil is liquid and coconut oil solid, or 35 degrees C where both oils are liquid. At 4 degrees C, rates of light degradation of all-trans retinyl palmitate in corn or coconut oil were significantly different from each other; at 35 degrees C they were not. Different droplet sizes of the carrier systems were achieved by homogenizing skim milk at different pressures (169, 105, or 35 kg/cm2). In all cases, greater loss occurred in corn oil as the vitamin carrier compared to coconut oil. Degradation rate of all-trans retinyl palmitate increased with decreased homogenization pressure. Effects of photooxidation of fatty acids on degradation of all-trans retinyl palmitate and isomerization of retinyl palmitate were investigated in four carrier systems (corn, butter, peanut, and coconut) by exposing the fortified oils to light. Peroxide values did not parallel degradation of all-trans retinyl palmitate during light exposure. Isomerization of all-trans to cis isomers of retinyl palmitate occurred in all carrier systems.