For finite adspecies mobility, the lattice-gas monomer-dimer (A+B2) surface reaction model exhibits a discontinuous transition from a stable reactive steady state to a stable A-poisoned steady state, as the impingement rate PA for A increases above a critical value P*. The reactive (poisoned) state is metastable for PA just above (below) P*. Increasing the surface mobility of Aenhances metastability, leading to bistability in the limit of high mobility. In the bistable region, the more stable state displaces the less stable one separated from it by a planar interface, with P*becoming the equistability point for the two states. This hydrodynamic regime can be described by reaction-diffusion equations (RDE’s). However, for finite reaction rates, mixed adlayers of A and Bare formed, resulting in a coverage-dependent and tensorial nature to chemical diffusion (even in the absence of interactions beyond site blocking). For equal mobility of adsorbed A and B, and finite reaction rate, the prediction for P* from such RDE’s, incorporating the appropriate description of chemical diffusion, is shown to coincide with that from kinetic Monte Carlo simulations for the lattice-gas model in the regime of high mobility. Behavior for this special case is compared with that for various other prescriptions of mobility, for both finite and infinite reaction rates.