In order to choose an adequate heat transfer fluid (HTF) for particular applications, it is necessary to know accurately the thermophysical properties, such as isobaric heat capacity, density, or viscosity. In this work, we have performed density and dynamic viscosity measurements of pure diphenyl ether and three different binary mixtures of diphenyl ether and biphenyl, including the eutectic composition, over a broad temperature range, i.e., from (288.15 to 373.15) K. A rotational Anton, Paar Stabinger SVM 3000 device was used to obtain these experimental values. Moreover, isobaric heat capacities have been measured for these samples in the liquid phase at temperatures from (293.15 to 353.15) K and pressures up to 25 MPa using a high-pressure flow calorimeter. From these measurements we have analysed how they depend on temperature and/or pressure. We have also determined the Prandtl, reference Grashof, and reference Rayleigh numbers. In addition, several equations were used to describe the temperature behaviour of viscosity and density, as well as the temperature and pressure dependences of the experimental isobaric heat capacities. (C) 2015 Elsevier Ltd. All rights reserved.