The molybdenum nitrogenase catalyzes the reduction of N2 to NH3 via the accumulation of 8 electrons and 8 protons. We have shown that electrons for dinitrogen reduction can be delivered photochemically to the catalytic MoFe protein component by CdS nanocrystals. In this study, we explored the balance between photoexcitation, electron transfer (ET), and hole transfer (HT) in CdS:MoFe protein reactions by evaluating transient populations of catalytic intermediates by electron paramagnetic resonance spectroscopy. This has allowed us to develop, for the first time, pre-steady-state kinetic models describing the dynamics of the En-state intermediates (n = # of electrons and protons) of the iron-molybdenum cofactor (FeMo-co). Analysis of our data with these models is revealing interesting details of the mechanism of photochemical N2reduction. First, the rate of catalytic electron delivery to the MoFe protein is limited by the efficiency of hole scavenging; second, photochemical reduction populates isomers of the hydride-bound intermediates that are not observed during steady-state turnover with the Fe-protein; and third, when the rate of electron delivery exceeds the rate of N2 activation, the FeMo-co accesses reduced states, which our results suggest have lower reactivity toward N2. Together, our work is tracking toward a comprehensive understanding of the requirements for efficient photochemical N2 reduction.