Quantum yields for formation of the triplet excited state and for formation of singlet molecular oxygen have been measured for various porphyrin derivatives in benzene or methanol solution under different concentrations of dissolved O-2. Measurements have been made for both steady-state and pulsed laser excitation, with the resultant singlet molecular oxygen being detected by photooxygenation reactions or time-resolved emission spectroscopy, respectively. In most cases, excellent agreement is observed between the two experimental techniques, allowing accurate determination of the quantum yields. These values may now be used as primary standards for actinometric measurements. For certain tetraphenylporphyrins in highly polarizable solvents, however, the photostationary state method indicated quantum yields for O-2((1) Delta(g)) production that were markedly higher than those obtained by laser-induced luminescence. This apparent discrepancy is explained in terms of the formation of a relatively long-lived, nonluminescent encounter complex formed between O-2((1) Delta(g)) and ground state sensitizer that can be intercepted by quenchers but not detected by time-resolved emission spectroscopy.