The thermodynamic influence of hydrotrope para-toluene sulfonic acid (pTSA), when present in water-natural gas-hydrate system, is reported here for the first time. Hydrotropes are aromatic compounds amphiphilic in nature, yet differ from classical surfactants with their small hydrophobic part and comparatively larger hydrophilic part. The presence of surfactants in hydrate systems were reported not to influence the equilibrium thermodynamics of hydrate formation, i.e., the presence of surfactants did not shift the pure water-gas-hydrate equilibrium curves. This work finds pTSA, a hydrotrope and a surface-active agent, to influence the hydrate forming thermodynamics. The water-gas-hydrate equilibrium curves for three different additive concentrations in water were obtained from isochoric 'temperature cycling' experiments in a state-of-the-art PVT cell. The hydrotrope aggregation behaviour in water was incorporated, for the first time, into the classical statistical thermodynamic hydrate equilibrium prediction model proposed by van der Waals and Platteeuw [Am. Inst. Chem. Eng. 32 (1986) 1321]. The micelle size, or aggregation number, of hydrotropes was taken as the only adjustable parameter in the model, which was adjusted according to the hydrate equilibrium experiment data to give a minimum average absolute deviation (AAD) fit between the model and the experiment values. A relationship was subsequently developed for the micelle size and PTSA concentration, making the scheme a priori prediction model for concentrations within the tested region. (C) 2004 Elsevier B.V. All rights reserved.