The instrumental response function (IRF) needed for the analyses of fluorescence time-correlated single-photon counting (TCSPC) experiments is usually determined by measuring the Rayleigh scattering (RS) of colloidal particles. Since RS is a one-photon process the response function obtained is referred to as a one-photon IRF (OP-IRF). Hence, in a two-photon excited (TPE) fluorescence experiment the hyper-Rayleigh scattering (HRS) might suit the determination of the two-photon IRF better [A. Habenicht, J. Hjelm, E. Mukhtar, F. Bergstrom, L.B.-angstrom. Johansson, Chem. Phys. Lett. 354 (2002) 3671. In the present study, we explore the importance of OP- and TP-IRF in fluorescence depolarisation studies. POPOP in ethanol and n-octanol is analysed when using OP- and TP-IRF. It is demonstrated that the TP-IRF yields significantly better statistics. (c) 2005 Elsevier B.V. All rights reserved.