The effect of the time-of-flight of the exciting and emitted photons on fluorescence decays is studied. Fluorescence decay laws are obtained for spherical and linear geometries. Experimental results for silica nanoparticles and for Rhodamine 101 solutions are analysed according to the developed model. It accounts well for the main features of the observations, the inclusion of photon propagation times being essential for a correct description of the fluorescence decays under the described circumstances. It is also shown that from the time dependence of fluorescence one can in principle determine the absorption or scattering coefficients of the sensed medium. (c) 2005 Elsevier B.V. All rights reserved.