The photodynamic activities of a porphyrin-C₆₀ dyad (P-C₆₀) and its metal complex with Zn(II) (ZnP-C₆₀) were compared with 5-(4-acetamidophenyl)-10,15,20-tris(4-methoxyphenyl)porphyrin (P), both in homogeneous medium-bearing photooxidizable substrates and in vitro on the Hep-2-human-larynx–carcinoma cell line. This study represents the first evaluation of dyads, with a high capacity to form a photoinduced charge-separated state, to act as agents to inactivate cells by photodynamic therapy (PDT). Absorption and fluorescence spectroscopic studies were performed in toluene and N,N-dimethylformamide (DMF). The emission of the porphyrin moiety in the dyads is strongly quenched by the attached fullerene C₆₀ moiety. The singlet molecular oxygen, O₂(¹Δ_g), productions (Φ_Δ) were determined using 9,10-dimethylanthracene (DMA). The values of Φ_Δ were strongly dependent on the solvent's polarity. Comparable Φ_Δ values were found for dyads and P in toluene, while O₂(¹Δ_g) production was significantly diminished for the dyads in DMF. In more polar solvent, the stabilization of charge-transfer state takes place, decreasing the efficiency of porphyrin triplet-state formation. Also, both dyads photosensitize the decomposition of L-tryptophan in DMF. In biological medium, no dark cytotoxicity was observed using sensitizer concentrations ≤1 μM and 24 h of incubation. The uptake of sensitizers into Hep-2 was studied using 1 μM of sensitizer and different times of incubation. Under these conditions, a value of ∼1.5 nmol/10⁶ cells was found between 4 and 24 h of incubation. The cell survival after irradiation of the cells with visible light was dependent upon light-exposure level. A higher photocytotoxic effect was observed for P-C₆₀, which inactivates 80% of cells after 15 min of irradiation. Moreover, both dyads keep a high photoactivity even under argon atmosphere. Thus, depending on the microenvironment where the sensitizer is localized, these compounds could produce biological photodamage through either an O₂(¹Δ_g)-mediated photoreaction process or a free-radicals mechanism under low oxygen concentration. These results show that molecular dyads, which can form a photoinduced charge-separated state, are a promising model for phototherapeutic agents, with potential applications in cell inactivation by PDT.