We analyze lattice–gas reaction–diffusion models which include spontaneous annihilation, autocatalytic creation, and diffusion of particles, and which incorporate the particle creation mechanisms of both Schloegl’s first and second models. For fixed particle diffusion or hop rate, adjusting the relative strength of these creation mechanisms induces a crossover between continuous and discontinuous transitions to a “poisoned” vacuum state. Kinetic Monte Carlo simulations are performed to map out the corresponding tricritical line as a function of hop rate. An analysis is also provided of the tricritical “epidemic exponent” for the case of no hopping. The phase diagram is also recovered qualitatively by applying mean-field and pair-approximations to the exact hierarchical form of the master equation for these models.