The monomer-monomer surface reaction model with an adsorbate interaction term is studied. An epidemic analysis of the poisoning times (t(p)) for small square lattices as a function of lattice edge length L and interaction strength alpha at the point of equal adsorption rates yields a dynamic scaling relation which describes the crossover between log-power-law and exponential behavior in L, and is able to fit the entire dependence of t(p) upon alpha and L. The phase transition is further explored by varying adsorption rates and is found to follow second-order kinetics. A mean-field approximation is introduced as a comparison for the numerical results. (C) 1997 American Institute of Physics.