Nanofluids, i.e., colloidal dispersions of nanoparticles in a base liquid (solvent), have received considerable attention in the last years due to their potential applications. One attractive feature of these systems is that their thermal conductivity can exceed the corresponding values of the base fluid and of the fluid with large particles of the same chemical composition. However, there is a lack of agreement between published results and the suggested mechanisms which explain the thermal conductivity enhancement. Here we show the possibilities of the inverse photopyroelectric method for the determination of the effective thermal effusivity of the system constituted by small ZnO nanoparticles dispersed in dimethyl sulfoxide, as a function of the nanoparticles volumetric fraction. Using a phenomenological model we estimated the thermal conductivity of these colloidal samples without observing any significant enhancement of this parameter above effective medium predictions. 2009 © The Japan Society for Analytical Chemistry.