We study a monomer-dimer model with repulsive interactions between the same species in one dimension. With infinitely strong interactions the model exhibits a continuous transition from a reactive phase to an inactive phase with two equivalent absorbing states. Static and dynamic Monte Carlo simulations show that the critical behavior at the transition is different from the conventional directed percolation universality class but is consistent with that of the models with the mass conservation of modulo 2. The values of static and dynamic critical exponents are compared with those of other models. We also show that the directed percolation universality class is recovered when a symmetry-breaking field is introduced.PACS numbers: 64.60.-i, 02.50.-r, 05.70.Ln, 82.65.Jv A monomer-dimer(MD) model was introduced by Ziff, Gulari, and Barshad to describe the oxidation of carbon monoxide on catalytic surface [1]. In this model, a monomer (CO) adsorbs onto a single vacant site, while a dimer (O 2 ) adsorbs onto a pair of adjacent vacant sites and then immediately dissociates. A nearest neighbor of adsorbates, comprised of a dissociated O atom and an CO atom, reacts and forms a CO 2 molecule and desorbs from the metal surface. In two dimensions, as the CO gas pressure is lowered, the system undergoes a first-order transition from a CO-saturated inactive phase into a reactive steady state and then a continuous transition into a O 2 -saturated inactive phase. This continuous transition is shown to be in the same universality class as the directed percolation (DP) [2,3,4]. In one dimension, there is only a first-order phase transition between two saturated phases.