A new series of phthalocyanine-containing biodegradable poly(is an element of-caprolactone)s have been synthesized by ring-opening polymerization of is an element of-caprolactone and/or 5-ethylene ketal is an element of-caprolactone using silicon phthalocyanine dihydroxide as the initiator and tin(II) 2-ethylhexanoate as the catalyst. The polymers have been spectroscopically characterized, and their molecular weights have been determined by gel permeation chromatography (GPC). All the polymers exhibit typical electronic absorption and fluorescence characteristics for nonaggregated phthalocyanines. The singlet oxygen quantum yields of selected polymers have been determined and found to be slightly lower than that of unsubstituted zinc(II) phthalocyanine in NN-dimethylformamide (DMF). Nanoparticles with an average hydrodynamic radius of similar to87 nm have been prepared from the homo-poly(is an element of-caprolactone) 3a and 3b (bulk) via a microphase inversion method. The ketal-containing analogues 5-7 are able to form nanoparticles with a smaller dimension ( = 26-46 nm) in the absence of surfactants. The biodegradation of these systems with Lipase PS has been monitored by a combination of static and dynamic laser light scattering together with fluorescence spectroscopy, which shows that phthalocyanine rings are released during degradation. The polymers exhibit a high phototoxicity toward the HepG2 cancer cell line. The results suggest that this novel polymer-based colloidal system is potentially useful for the delivery and release of photo-sensitizers in photodynamic therapy.