The faradaic admittance for a consecutive E-C-E mechanism with two stable intermediates is derived with the assumption that the chemical reaction is unidirectional. Both cases where this reaction is homogeneous or heterogeneous are treated. The admittance derived assuming that the chemical step is slow in comparison with the frequency of the a.c. perturbation has the same frequency dependence as that derived by Armstrong and Firman for a consecutive E-E mechanism with a stable intermediate. The presence of the chemical step in the E-C-E mechanism appears in the altered potential dependence of the admittance data. Four cases are distinguished in which the general admittance expression reduces to the simpler Bandies expression (pseudo-Randles cases). The theory is applied to the reduction of nitromethane in aqueous 1 M NaCl solutions of pH 2.4. Several methods of analysis are proposed. It is found that a simultaneous analysis of the frequency and potential dependence of the admittance data is to be preferred compared with the habitual procedure of frequency analysis at successive fixed potentials. The final reaction parameters obtained from the analysis are discussed in comparison with previous data obtained in basic media, which conform well to an E-Ce mechanism with only one stable intermediate.