It is shown that radiation propagation may affect time-resolved fluorescence measurements involving millimetre-sized systems and picosecond time scales. For this purpose, the effect of the excitation beam width on the instrument response function is studied. Experimental results for a scattering suspension of silica nanoparticles in a square 1 cm cell are analysed according to the developed model. The model accounts well for the observations, the retrieved parameters being in very good agreement with expectations. (c) 2006 Elsevier B.V. All rights reserved.