The hypersound velocity in a series of binary mixtures containing the constituents 4-n-pentyl-4′-cyanobiphenyl and 4-cyano-4′(2-methyl)butoxybiphenyl has been measured using Brillouin scattering techniques. Measurements have been made in the isotropic phase of the mixtures over the temperature range 30°C to 80°C. A simple theory for the observed velocity variations is proposed and compared with the experimental data. Agreement is found to be excellent over the temperature range studied.