Based upon a hitherto undescribed difference in physical behavior of the coproporphyrin I and III esters[long dash]namely, the marked difference in rate of precipitation in 30% acetone when the range of concn. of the total coproporphyrin is from 25-70 [gamma]/IOO ml. Since as little as 10 ml. of final soln. is required, it is evident that the method is applicable to the resolution of mixtures of from 3-7 [gamma] of total coproporphyrin. The difference in rate of precipitation is made evident by the rapid disappearance of red fluorescence in Wood''s light in the case of the type I isomer, while with the type III isomer the disappearance is negligible within 1 1/2 hrs.