The photophysical characteristics of two second-generation PDT photosensitizers, tin ethyl etiopurpurin I (SnET2) and tin octaethylbenzochlorin (SnOEBC), have been measured in homogeneous solution and when bound to bovine serum albumin (BSA). The ground state and triplet state absorption spectra have been characterized, as have triplet lifetimes and quantum yields for intersystem crossing, singlet oxygen formation and photobleaching. In total, these parameters provide a complete set of data that can be used to quantitatively compare the photosensitizing efficiencies of these molecules. The photo-bleaching quantum yield of SnET2 is increased dramatically when it is bound to BSA, thus limiting the production of singlet oxygen at incident fluences above 1 J/cm2. In contrast, the quantum yield of photobleaching of SnOEBC is at least an order of magnitude lower than that of SnET2 under these conditions and does not significantly limit the photosensitization process for typical in vivo or in vitro fluences. This difference is expected to play a significant role in determining the relative photo-sensitizing ability of these compounds in vivo.