An irreversible surface reaction with diffusion is studied by Monte Carlo simulation. The exponents at the poisoning transition are found to be the same as their values in the absence of diffusion on the surface. The phase diagram is not altered. The average poisoning time for either species, as a function of their relative concentration x, diverges as ‖0.5−x‖−γ, with γ=0.9±0.1. The surface coverage by either species grows linearly for small times; for much longer times the majority species saturates as 1−exp(−const⋅t). The power spectrum of the fluctuations is Gaussian at the transition point.