Ferrouids are colloids of superparamagnetic nanoparticles that are envisaged for use in hyperthermia, which is based on nonradiative relaxation after interaction with a high-frequency magnetic eld or light. For such applications, an important parameter is the thermal diusivity. In this communication, we present an experimental study of the dependence of thermal diusivity of ferrouids on the size of the magnetite nanoparticles by employing the mode-mismatched thermal lens technique. The results show a huge enhancement of the thermal diusivity by increasing the average size of the nanoparticles, while the number density of the nanoparticles is maintained as constant.