Simple models of bimolecular surface reactions occurring on a regular lattice are investigated by means of Monte-Carlo simulations. Fractal kinetics, observed in various steady-state regimes, appear to be due to the fractal nature of the adsorbate clusters generated by the microscopic mechanisms of the kinetic scheme. Two-site mechanism rates involve anomalous orders which are tentatively assigned to cluster properties defined in terms of ad-percolation specifics.