SU5416 (Z-isomer), the first in its class of angiogenesis inhibitors, in solution converts to the E-isomer following light exposure and reverts to the Z-isomer in the dark. Kinetics of this Z-E isomerism in pharmaceutical media is reported. Analytical solutions need light protection at 5 degrees C to maintain integrity. While E-isomer in light-exposed product increased to 0.9% in 24 hours, light-protected product showed no change (25 degrees C, 18 months). Infusate studies indicated that < 1.9% E-isomer will be dosed to patients and would likely convert to the Z-isomer, following administration. This report implies Z-E isomerism in SU5416 is controllable with no limitations towards ensuring pharmaceutical product quality.