A theoretical study of the EC(1irr) reaction is presented, where the reactants can be adsorbed; it also applies to EC(1irr)E reactions. The apparent nature of the chemical reaction C-1irr is determined by the apparent nature of the electrochemical reaction E. If E appears as a heterogeneous process, C-1irr appears as homogeneous (rate constant kappa(happ)); if E is a surface reaction, C-1irr appears as a surface reaction (rate constant kappa(sapp)) However, the significance of the constants is complex. For example, kappa(happ) can be the true homogeneous constant kappa(h), or an apparent constant whose value is determined solely by the true surface constant kappa(s). Similarly, kappa(sapp) can be the true surface constant kappa(s), or an apparent constant whose value is determined solely by the true homogeneous constant kappa(h). In each case, there exists no intrinsic discrimination between the two possibilities; only extrinsic factors, such as the variations of the constants with pH, or the use of spectroelectrochemical methods, can help to establish the actual nature of the constant, although the true surface constant is most probably obtained when all the components of the reaction are strongly adsorbed. Several experimental cases, in particular the benzidine rearrangement, are examined in the light of theory.