The Fourier path integral formalism was implemented in the grand canonical ensemble and applied to a two-dimensional Lennard-Jones fluid, which is used to model a quantum monolayer. The method was employed to obtain the variation in the density of the monolayer as a function of chemical activity at a fixed temperature. Also, the heat of adsorption was calculated as a function of chemical activity using a modified virial estimator to compute the total energy. The results obtained from the adsorption isotherms suggest that the quantum contribution causes a decrease in the amount of adsorbed material and destabilization of the monolayer. (C) 2003 Elsevier Science B.V. All rights reserved.