The aim of this paper is to determine the site-occupancy pair correlation functions of a partially ordered layer of CO adsorbed on Pt(111). These functions are needed to calculate the structure factor appearing in the diffuse LEED intensity expression. CO molecules can be adsorbed at on-top sites and at three kinds of bridge sites obtained from each other by rotation of 120°. Pair correlation functions Γ i j (p,p ) = n i (p)n j (p ) - n i (p) n j (p ) are obtained by a Monte Carlo simulation at constant coverage m and temperature T. The model involves adsorption energy and repulsive pair interaction potentials up to third neighbours, depending only on the distance. We deal with a N N fcc (111) 2D lattice with periodic boundary conditions. We study the dependence of correlation functions on pair interaction potentials and coverage m . The determination of correlation functions is based on the experimental observations coming from the HREELS, IRAS and LEED techniques. In accordance with experiment, a diffuse (√3 √3)R30° superstructure, involving only on-top sites, appears at low coverage. On increasing the coverage, more and more bridge sites are occupied and a c(4 2) superstructure grows progressively.