The molecular inorganic compound 1,3,5-{trans-[RuCl(dppe)(2)]Cequivalent toC-4-C(6)H(4)Cequivalent toC}(3)C6H3 (1) is reversibly oxidized in solution using an optically transparent thin-layer electrochemical (OTTLE) cell, the oxidation to 1(3+) resulting in the appearance of a strong absorption band at 11 200 cm(-1) in contrast to 1, which is optically transparent at frequencies below 20 000 cm(-1). This linear electrochromic behavior is accompanied by third-order nonlinear optical electrochromic behavior, which has been probed by a combination of femtosecond degenerate four-wave mixing (DFWM) and pump-probe experiments at 12 500 cm(-1). DFWM studies of 1 and 1(3+) reveal an order of magnitude increase in the hyperpolarizability, \gamma\, upon oxidation with \gamma\ values of 2000 x 10(-36) esu and 20 000 x 10(-36) esu, respectively; the DFWM signal of the latter is dominated by a delayed response (relaxation time ca. 1 ps). The transient absorption (TA) data for 1 reveal efficient two-photon absorption (sigma(2) approximate to 10(3) GM), whereas the TA data for 1(3+) reveal saturable absorption (decay time ca. 1 ps). The switching of third-order nonlinearity in the 1/1(3+) pair represents the first demonstration of femtosecond time-scale processes being responsible for nonlinear electrochromism.