The aim of this study was to characterize the kinetics of metabolite formation of the phosphodiesterase type-5 (PDE5) inhibitors sildenafil and tadalafil by CYP3A4, CYP3A5, and CYP3A7 isoforms. The formations of N-desmethyl sildenafil and desmethylene tadalafil were examined using CYP3A supersomes co-expressing human P450 oxidoreductase and cytochrome b5. Both sildenafil N-demethylation and tadalafil demethylenation were catalyzed by CYP3A4, CYP3A5, and to a lesser extent by CYP3A7. The kinetics of desalkyl metabolite formation of the two drugs were well fitted to the Hill equation; however, the Hill coefficients (n) suggested CYP3A-mediated negative cooperativity. Next, we analyzed the kinetics with a two binding sites model assuming two reaction steps: reaction 1 with high-affinity and low-capacity metabolism and reaction 2 with low-affinity and high-capacity metabolism. The kinetics of desalkyl metabolite formation were also fitted to the two binding sites model. The intrinsic clearance (CLint) values of reactions 1 and 2 for sildenafil N-demethylation were 0.733 and 0.033 µL/min/pmol P450 for CYP3A4, 0.788 and 0.019 µL/min/pmol P450 for CYP3A5, and 0.079 and 0.004 µL/min/pmol P450 for CYP3A7, respectively. The CLint values of reactions 1 and 2 for tadalafil demethylenation were 0.187 and 0.014 µL/min/pmol P450 for CYP3A4, 0.050 and <0.001 µL/min/pmol P450 for CYP3A5, and 0.004 and <0.001 µL/min/pmol P450 for CYP3A7, respectively. These results may provide the basis not only for understanding the metabolic properties of the two PDE5 inhibitors, but also for one possible explanation of the mechanisms of CYP3A-mediated negative cooperativity.