A new type of modulation fluorometer was used in the study of energy-dependent chlorophyll fluorescence quenching (qE) in intact leaves. Under conditions of strong energization of the thylakoid membrane (high light intensity, absence of CO2) not only variable fluorescence, FV, but also dark-level fluorescence, FO, was quenched, leading to definition of a quenching coefficient, qO. Information on qO was shown to be essential for correct determination of photochemical (qQ) and energy dependent quenching (qE) by the saturation pulse method. The relationship between qE and qO was analysed over a range of light intensities at steady state conditions. qE was found to consist of two components, the second of which is linearly correlated with qO. qO and the second component of qE are interpreted to reflect the state 1 — state 2 shift caused by LHC II phosphorylation.