We compute second order derivatives of the Gibbs energy by Monte Carlo simulation in the isobaricÈisothermal ensemble for Ñuids made of rigid and Ñexible molecules and test the accuracy of the simple interactions potential. The thermal expansivity and the isothermal compressibility can be calculated directly during a simulation run. The total heat capacity is obtained as the sum of the residual heat capacity computed using the Ñuctuation method and the ideal heat capacity, which cannot be determined by Monte Carlo simulation and must be taken from experimental data. The JouleÈThomson coefficient is obtained by the combined use of thermal expansivity and total heat capacity. The Ñuctuation method proves to converge very well, with limitation at low pressure for the JouleÈThomson coefficient. The Ñuctuation method has been extensively tested on pure light hydrocarbons (methane, ethane and butane) in the vapour and liquid states. In the case of methane, we used a united atom Lennard-Jones potential (D.J. Oprzynski, A. and J. Mo ller, Mu ller Fischer, Mol. Phys., 1992, 75, 363). Detailed comparison with experimental heat capacities, volumetric properties and JouleÈThomson coefficients at pressures up to 100 MPa showed excellent agreement. The inversion of the JouleÈThomson e †ect is predicted with an excellent accuracy. In the case of ethane and n-butane, we used an anisotropic united atoms potential (P. Ungerer, C.

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