A simple photochemical model for the photosynthetic units of Photosystem II based on first-order rate constants for de-excitation of excited chlorophyll molecules is presented in the form of equations which predict the yields of fluorescence (i.e. at the FO level, at the maximal FM level and the fluorescence of variable yield, FV equals FM minus FO). Two types of quenching mechanisms are recognized: (1) increasing nonradiative decay processes in the bulk chlorophyll by creating quenching centers which complete with the reaction centers for the excitation energy (this mechanism quenches both FO and FV) and (2) increasing nonradiative decay of the excited reaction center chlorophyll (this mechanism quenches FV but not FO). Quenching in the bulk chlorophyll preserves the relationship that Fv/FM is equal to the maximum yield of photochemistry; quenching at the reaction center chlorophyll decreases FV/FM substantially (since FV is quenched specifically) but may have very little effect on the yield of photochemistry. Estimates are made of the relative magnitudes of the rate constants for de-excitation of the excited reaction center chlorophyll by photochemistry, kp, by nonradiative decay processes, kd, and by energy transfer back to the bulk chlorophyll, kt. Fluorescence is assumed to emanate only from the bulk chlorophyll. Energy transfer from Photosystem II to Photosystem I may occur from either the excited bulk chlorophyll or from the excited reaction center chlorophyll. The model is valid for any degree of energy transfer between Photosystem II units.