Abstract: Carboxyhydroxyporphin compounds such as hematoporphyrin exhibit instability upon warming. However, if this warming is extended and carried out in solution in the presence of a buffer so as to constitute a moderate temperature heat-treatment, a new composition of matter results which contains as much at 95% or more of an ester-linked oligomeric porphin-based material which is exceedingly storage-stable. This composition has superior activity as a photoimaging and photodynamic therapy agent. This process and the products it produces are disclosed. The oligomer products so formed can be formulated into pharmaceutical compositions for use in neoplastic tissue photoimaging and in photodynamic therapy of neoplastic tissues.