The two constituent phases of the nanofluids have thermal expansion coefficients that are significantly different. Moreover, the variability of the thermal expansion coefficients of fluids with temperature is significantly higher than that of solid materials. The mismatch of the thermal expansion coefficients creates changes of the volumetric fraction of solids with temperature changes. The changes can be significant with fluids that have high thermal expansion coefficients, such as refrigerants and fluids that operate close to their critical points. Since the thermal conductivity of nanofluids is a very strong function of the volumetric fraction of the nanoparticles, these changes of the volumetric fraction may cause significant effects on the thermal conductivity of the nanofluids, which must be accounted for in any design process.