Spectroelectrochemistry using a cell with a short conversion time of about 2 s and fast scan cyclic voltammetry permits us to observe the radical anions of 4-nitro- and 4-cyano-phenylazo tert-butyl sulfides and to measure the electrocatalyzed (Z) to (E) stereomutation. The rate of this fast isomerization is of the order of 10(6) s(-1). It was also possible to characterize kinetically the photostationary state obtained upon light irradiation.