The present study was undertaken to find new ways to improve efficacy of photodynamic therapy (PDT). We investigated the combinatory effect of the photosensitizer Photofrin and ursodeoxycholic acid (UDCA). UDCA is a relatively non-toxic bile acid which is used inter alia as a treatment for cholestatic disorders and was reported to enhance PDT efficiency of two other photosensitizers. Since besides necrosis and autophagic processes apoptosis has been found to be a prominent form of cell death in response to PDT for many cells in culture, several appropriate tests, such as cytochrome c release, caspase activation and DNA fragmentation were performed. Furthermore spectral resolved fluorescence lifetime imaging (SLIM) was used to analyse the cellular composition of Photofrin and the status of the enzymes of the respiratory chain. Our experiments with two human hepatoblastoma cell lines revealed that the combination of Photofrin with UDCA significantly enhanced efficacy of PDT for both cell lines even though the underlying molecular mechanism for the mode of action of Photofrin seems to be different to some extent. In HepG2 cells cell death was clearly the consequence of mitochondrial disturbance as shown by cytochrome c release and DNA fragmentation, whereas in Huh7 cells these features were not observed. Other mechanisms seem to be more important in this case. One reason for the enhanced PDT effect when UDCA is also applied could be that UDCA destabilizes the mitochondrial membrane. This could be concluded from the fluorescence lifetime of the respiratory chain enzymes which turned out to be longer in the presence of UDCA in HepG2 cells, suggesting a perturbation of the mitochondrial membrane. The threshold at which PDT damages the mitochondrial membrane was therefore lower and correlated with the enhanced cytochrome c release observed post PDT. Thus enforced photodamage leads to a higher loss of cell viability.