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 initial F 0 level, at the maximal F M level and the fluorescence of variable yield, F V = F M − F 0). 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 F 0 and F V) and (2) increasing nonradiative decay of the excited reaction center chlorophyll (this mechanism quenches F V but not F 0). Quenching in the bulk chlorophyll preserves the relationship that F V F M is equal to the maximum yield of photochemistry; quenching at the reaction center chlorophyll decreases F V F M substantially (since F V 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, k p, by nonradiative decay processes, k d, and by energy transfer back to the bulk chlorophyll, k t. 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.