The heterocyclic amines 2,6-lutidine, pyrazine, piperazine and piperidine were intercalated into layered crystalline hydrated barium phenylphosphonate, Ba(HO3PC6H5)(2)center dot H2O, through a batch method in ethanolic solution, to give the maximum amounts 0.39, 0.82, 2.80 and 5.50 mmol g(-1), respectively. The original host interlayer distance (d) of 1532 put increased after intercalation for piperazine (1752 pm) and piperidine (2112 pm) molecules, while for 2,6-lutidine and pyrazine molecules d values were maintained. The enthalpy of intercalation gave -5.60 +/- 0.10, -1.00 +/- 0.02, -9.55 +/- 1.00 and -30.70 +/- 0.68 kJ mol(-1) for the sequence of heterocyclic amines. The Gibbs free energies are negative and entropies are positive for intercalation. (c) 2005 Elsevier B.V. All rights reserved.