In this work, we investigate the influence of energetical lattice heterogeneities on the finite-size scaling behavior of the tracer diffusion coefficient in the vicinity of a critical point. For this purpuse, we study a lattice gas with pairwise repulsive nearest-neighbor (NN) interactions on a square bivariate trap surface, with deep trap concentrations ranging from 0 to 5%. This well-known and frequently studied system exhibits c(2 x 2) ordering below the critical temperature T-c = 0.567 phi(NN)/k(B). We show that the presence of a small amount of deep adsorption sites (traps) strongly influences adsorbate diffusion near the phase transition. D-t depends onthelattice size L of the lattice near T,. For all values of L studied, D-t(T)(0=0.5) is a smooth function and exhibits an inflexion point at or near T-c. Its derivative, partial derivativeD(t)/partial derivative(1/k(B)T), exhibits cusp-like maxima. The behavior of these maxima is analyzed in terms of finite-size scaling theory. (C) 2002 Elsevier Science B.V. All rights reserved.