The interaction between phthalocyanine (ZnPc) and poly(ethylene oxide) (PEO) in water has been studied by dynamic laser light scattering and transient absorption and fluorescence spectroscopies. After the addition of PEO, the intensity of the transient absorption at ca. 490 nm, arising from the triplet state of the nonaggregated ZnPc, increases, and the triplet lifetime becomes longer. These results together with the fluorescence quenching experiments reveal that PEO behaves as a dispersion agent to disstack phthalocyanine macrocycles into individual molecules. It has been found that an equimolar amount of long PEO chains (M-w similar to 10 000 g mol(-1)) can completely disstack ZnPc aggregates in water. A long PEO chain is about 8 times more effective than its short counterpart (M-w similar to 2000 g mol(-1)). The disstacking process has also been monitored by dynamic light scattering, which shows that the average particle size decreases significantly upon the addition of PEO. Since PEO is a FDA-certified biocompatible polymer material, such a phthalocyanine system has a potential application in photodynamic therapy.