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, dp = 50 nm) are made at volume fractions from 1-5 vol% over the temperature range 20-90°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, are used to determine