A numerical model is designed to model transient cyclic voltammetry (CV) on electroactive biofilms. The dependence of the transient current peak (J_peak) on the potential scan rate (v) is approached through a power law (J_peakvs. v^α) as is usually done in experimental studies. The two straightforward rules of thumb (α = 1 or α = 0.5), which are the only theoretical tools available so far, are shown to be partly deficient. In contrast, the model explains the fact that the α exponent can vary in a large range of values from 1 to 0.34 (possibly lower), as observed in experimental studies, and gives theoretically supported rules for interpreting transient CV of electroactive biofilms.