Reversible switching of the molecular second-order nonlinear optical (NLO) polarizability has been achieved by means of proton-transfer. The NLO molecule is an azafulleroid, based on a homoconjugated fullerene moiety as the acceptor as a new attractive building block for organic NLO materials. The electron-donor (proton-acceptor) group is the dimethylamino group. The NLO response was dramatically reduced upon protonation, and completely restored after deprotonation. Analogies and differences between this proton-transfer based switching and earlier reported electron-transfer based switching are discussed, along with the implications for third-order NLO materials based on a composite material comprised of a second-order NLO molecule and a photoaddressable proton donor. (C) 2002 Elsevier Science B.V. All rights reserved.