Optical sensors based on the luminescence lifetime quenching are often considered as being more attractive than luminescence intensity-based sensors. However, depending on the nature of the luminophore-matrix system considered, the relative positions of the I0/I and τ0/τ curves as a function of the quencher concentration can vary considerably. With a view to rationalize these observations, numerical computations of the I0/I and τ0/τ ratios have been performed in a two-site model. It is shown that this simple model can explain the various observations and that lifetime measurements performed by phase fluorometry are sometimes not suitable for analytical application.