The theory of adsorption of macroscopic systems on nonuniform surfaces has been developed in consid erable detail using the lattice gas model (LGM) [1]. Lateral interactions between adsorbed molecules and surface nonuniformity strongly influence equilibrium adsorption characteristics (isotherms and heats of adsorption). For microcrystals, such studies are at the beginning stage. It was shown in [2-5] that density fluctuation effects become important for adsorption on small sized crystalline particles, and their role is fairly noticeable for nonuniform surfaces. The influ ence of intermolecular interactions on adsorption iso therms has currently been considered in the mean field approximation only [3,[6][7][8][9]. This approximation fairly roughly described intermolecular interactions, and, in this work, more exact equations were con structed for taking interactions of molecules into account in the quasi chemical approximation describ ing direct correlations between molecules.Let us consider a nonuniform crystal surface and describe adsorption using the LGM. We will analyze the distribution of molecules in a grand canonical ensemble (μ, М, T), where μ is the chemical potential of molecules in the volume phase; М is the total num ber of surface nodes consisting of regions with size M q ,; and t is the number of node types on the surface of a microcrystal. Each node can be either occupied or free. Let N q be the number of adsorbed molecules on nodes of type q. The number of free nodes of type q will be denoted byThe mean number of pairs of nodes of different types qp will be denoted by M qp . Pairs of nodes of dif ferent types are related as , where z q is the number of neighbors of a node of type q, and prime at the sum sign means the absence of the term with p = q, 1 ≤ p ≤ t. Let z qp be the number of nodes of type p neighboring nodes of type q. These numbers char acterize the local structure of a nonuniform surface. Their relation to the numbers of node pairs M qp of the qp type can be written as = or = , where is the Kronecker symbol. The total balance of pairs of nodes is written in the form of two sums, + .Let J q be the partition function of a particle in a node of type q, 1 ≤ q ≤ t, and J the partition function of a molecule in the gas phase. The other denotations are: μ = β -1 ln(βP/J), β = (kT) -1 , P is the gas phase pres sure, and ε q is the bond energy between a particle and a node of type q on the surface of the adsorbent. Lat eral interaction parameters ε qp depend on the type of the qp pair, 1 ≤ q, р ≤ t, on which two neighboring mol ecules are adsorbed.The equation for the partition function of a non uniform system in the quasi chemical approximation with taking into account interaction between the near est neighbors only is written as , where, .1 ' 2 t qp qq q q p M M z M = + = ∑ qp z / (1 ) qp qp q M M + Δ qp M /(1 ) qp q qp z M + Δ qp Δ 1 t qq q M = ∑ * 1 t qp qp M = ∑ 1 2 Q Q Q = , 1 , 0 1 q q q q q q t M M qq N N q Q Q = = = ∑ ∏ * 2 0 1 qp qp t M qp N qp Q Q = = = ∑ ∏ Abstract-Equations for the...