We have studied the solubility of a small probe particle, the atoms He and Ar anti the molecule CH4 in the helical poly(alpha-alkyl-beta-L-aspartate)s using molecular simulations. The hexagonal and tetragonal crystal forms of poly(alpha-n-butyl-beta- L-aspartate) and the hexagonal form of the poly(alpha-methyl-beta-L-aspartate) have been chosen for the study as the most representative structures for this family of polymers. An important characteristic of these structures is that the helical conformations are retained at high temperatures. The excess chemical potentials have been determined using Widom＇s test-particle insertion method considering an ensemble of microstructures generated by Monte Carlo calculations. The solubility of the different penetrants in the polymer systems was explained in terms of the distribution of the unoccupied space. (C) 2000 Elsevier Science Ltd. All rights reserved.