In situ reversible electrochemical switching of the molecular second-order nonlinear optical (NLO) polarizability, or first hyperpolarizability, has been implemented in a specially designed cell. The redox-switchable NLO chromophore is based on the octamethylferrocene/octamethylferrocenium redox system as electron-donor (D) group, in conjunction with nitrothiophene as the electron-acceptor (A) group and ethenyl as the pi-conjugation bridge. This D-pi-A chromophore has been shown to exhibit reversible redox switching of its linear and nonlinear optical properties. The importance and potential of this electrochemical switching of the first hyperpolarizability is discussed in the context of current and future applications of second-order NLO effects. (C) 2002 Elsevier Science B.V. All rights reserved.