A storage stable porfimer sodium (polyhematoporphyrin ether/ester) composition useful in the photodynamic therapy of cancer and other conditions is provided. The composition is characterized in that the percentage of ester linked porphyrin oligomers is less than 10% of the composition. Improved processes for preparing such compositions are also provided wherein acetylated hematoporphyrin is treated with alkali for sufficient time and temperature to reduce the amount of porphyrin oligomers joined by ester linkages to less than 10% of the composition.