Introduction Second-order thermodynamic properties, such as compressibility, thermal expansion coefficients and heat capacity, play an important role in the oil and gas industry. The knowledge of the volumetric behaviour of reservoir fluids is essential during production and transport operations whereas heat capacity C P is an essential property to perform thermal calculations in all heat transfer processes. Moreover amongst these second order properties heat capacities occupies a special place due to its relation with entropy, i.e. C p =T(¶S/¶T) p which provide information on the microscopic structure of the system. As a result, the accurate measurement and comprehensive modeling of these second-order properties for liquids and their mixtures are of interest to both the improvement of our knowledge of liquid structure and to optimization of many processes in oil and gas engineering. It is well known that most classical equations of state (EOS) present serious limitations for describing second-order properties the main reason for this being that they have been developed to describe phase equilibria [1, 2]. As long as the complexity of the system, as for instance the molecular chain increases which is the case of components which belong to the heavy fraction of the crude oil, additional shortcomings appear. In this context of special relevance is the work of Llovell et al. [3], Vijande et al. [4] and Lafitte et al. [5] which propose a complete thermo- dynamic description i.e. phase equilibria and thermo- dynamic behavior from a single approach using the same molecular EOS, namely the SAFT in different versions. In addition it has been demonstrated [5] the interest of adding thermodynamic properties namely densities and speed of sound to the classical fitting procedure based on the LV equilibrium. This procedure yields between accuracies of the estimated properties and at the same time, parameters with a stronger physical meaning. In view of the application of these models to complex multi-components systems like reservoir fluids, it is of primordial to have large database for pure system in hand to test and to develop, the models. In this context, the main goal of this work is to provide reliable data of thermodynamic properties and their temperature and pressure dependence. To this end, attention was focused on the experimental determination of molar isobaric heat capacities C p,m (P,T) and speed of sound c(P,T) for n-undecane, a normal alkane which belongs to the above mentioned heavy fraction. Then, using this set of data, density, isentropic and isothermal compressibility and isochoric heat capacity were determined in the same [p,T] ranges. The consistency between calorimetric, acoustic and volumetric data is then checked [6]. Experimental Materials n-undecane was supplied from Fluka (purity, 99 mol%) and used without further purification. 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2007 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 89 (2007) 1, 81–85 THERMODYNAMIC CONSISTENCY BETWEEN CALORIMETRIC ACOUSTIC AND VOLUMETRIC MEASUREMENTS Application to n-undecane D. BessiÀres * and F. Plantier Laboratoire des Fluides Complexes, UMR CNRS 5150, Facult¾ des Sciences, Universit¾ de Pau, BP 1155, 64013 Pau Cedex, France In this work, experimental measurements of isobaric heat capacity as well as speed of sound were performed in the compressed liquid phase of n-undecane from 303.15 to 373.15 K and for pressures ranging up to 60 MPa. These results were used to estimate various thermophysical properties such as density, isentropic compressibility and isochoric heat capacity in the same ranges of pressure and temperature. All these sets of data allow checking the thermodynamic consistency between calorimetric, acoustic and volumetric properties. Keywords: heat capacity, isentropic and isothermal compressibilities, speed of sound, n-undecane * Author for correspondence: david.bessieres@univ-pau.fr