Fluorescence parameters obtained during steady-state electron transport are frequently used to evaluate photosynthetic efficiency of plants. We studied the behaviour of those parameters as a function of irradiance-adapted fluorescence yields FS and F'M. Applied simulations showed that photochemical quenching evaluated by qP is greatly influenced by the steady-state fluorescence level (FS), and that its evolution is not complementary to non-photochemical quenching (qN). On the other hand, the relative photochemical and non-photochemical quenching coefficients (qP(rel) and qN(rel)) proposed by Buschmann (1995) represent better the balance between the energy dissipation pathways. However, these relative parameters are also non-linearly related when the FS level is varied. We investigated the application of a new parameter, the relative unquenched fluorescence (UQF(rel)) which takes into account the fraction of non-quenched fluorescence yield (FS), which is related to closed photosystem 2 reaction centres not participating in electron transport. By using computer simulations and real in vivo measurements, we found that this new parameter is complementary to qP(rel) and qN(rel), which may facilitate the use of PAM fluorescence as diagnostic tool in environmental studies. and P. Juneau, B. R. Green, P. J. Harrison.