This study determines the thermophysical properties of nanofluids using ultrasonic techniques. Using an acoustic test cell, fitted with 4 MHz high-temperature transducers, measurements of the speed of sound in an aqueous dispersion of alumina nanoparticles (Al2O3, 99.9%, spherical, d(p) = 50 nm) are made at volume fractions from 1 to 5 vol % over the temperature range of 20-90 degrees C. The observed relationships between the measured parameters and speed of sound variation are presented. Available theoretical approaches are reviewed and applied to the data of the study. The speed of sound data together with measurements of density and predictions of thermal conductivity, derived from Lagrangian particle tracking (LPT) simulations, is used to determine the ratio of specific heats of nanofluids using a modified version of the Bridgman equation. The results demonstrate the effectiveness of the measurement technique, with outcomes elucidating the dependence of the speed of sound on temperature and particle concentration, and hence the influence of these parameters on the thermophysical properties of nanofluids. Using the speed of sound approach and LPT simulations, the predicted thermal values, which have an estimated accuracy of 5-10%, show good agreement with theoretical and experimental results available in the literature for similar operating conditions. This research forms the basis for the use of novel acoustic techniques for online, in situ measurement of nanofluids, and their potential applications in solar thermal power systems.