Layered crystalline hydrated barium phenylphosphonate, Ba(HO3PC6H5)(2) . H2O compound was employed as host for aromatic antines pyridine and alpha-, beta- and gamma-picolines, intercalation by using the batch method in ethanol at 298 I K, gave maximum amounts of (1.13, 0.76, 0.84 and 0.97) mmol . g(-1), respectively. The original interlamellar distance of 1532 pm increased after all intercalations. The thermochemical data were obtained on a differential isothermic microcalorimetric instrument by means of a titration procedure. From thermal effect data, the variation in enthalpy of intercalation for each system gave exothermic values of (-43.55 +/- 0.32, -2.36 +/- 0.04, -2.61 +/- 0.01 and -3.07 +/- 0.05) kJ . mol(-1) for the above sequence. The variation in enthalpic values for pyridine derivatives can be interpreted through the influence of steric hindrance associated with the methyl group attached to the heterocyclic ring. In addition to exothermic enthalpy, the spontaneous negative Gibbs free energy and the positive entropic values indicate favorable thermodynamic data for such intercalations. (C) 2004 Elsevier Ltd. All rights reserved.