Rate constants of elementary steps in the addition-elimination mechanism of nucleophilic vinylic substitutions (SNV) were determined by studying the following reactions in 50% Me(2)SO-50% water at 20 degrees C : (1) alpha-nitro-beta-(2,2,2-trifluoroethoxy)stilbene (Ph(OCH2CF3)C=C(NO2)Ph, 1-OTFE) and alpha,beta-dinitrostilbene (Ph-(NO2)C=C(NO2)Ph, 1-NO2) with OH-, piperidine, and HOCH2CH2S-, which led to the expected substitution products 1-OH, 1-Pip, and 1-SCH2CH2OH, respectively, although 1-OH is rapidly converted into PhC(O)C(Ph)=NO2- (3(-)); (2) 1-OTFE with CF3CH2O-, which leads to the adduct Ph(OCH2CF3)(2)CC(Ph)=NO2-, (2-(OTFE)(2)(-)); (3) 1-NO2 with CF3CH2O-, which leads to 3(-) by rapid hydrolysis of 1-OTFE formed as the immediate product of the reaction; (4) beta-methoxy-alpha-nitrostilbene (1-OMe) with CF3CH2O- which leads to the intermediate Ph(OCH3)(OCH2CF3)CC(Ph)=NO2- (2(OMe,OTFE)(-)); and (5) 1,2-diphenyl-2-nitroethanone 2,2,2-trifluoroethyl acetal (Ph(OCH2CF3)(2)CCH(Ph)(NO2)) with OH-, which leads to 3(-) by rapid hydrolysis of 1-OTFE, formed as the immediate product by CF3CH2O- loss from the deprotonated acetal. In the reactions of 1-OTFE with OH- and piperidine, as well as the reactions of 1-NO2 with OH-, CF3CH2O-, piperidine, and HOCH2CH2S-, the SNV intermediate, whose formation is the rate-limiting step, does not accumulate to detectable levels. However, in the reaction of 1-OTFE with HOCH2CH2S-, the intermediate Ph(OCH2CF3)(SCH2CH2OH)CC(Ph)=NO2-, (Z(OTFE,SR)(-)) is directly observable, which allows a kinetic determination of all steps in the reaction. The dependence of the various rate constants on structure can be understood as arising from an interplay of electronic, steric, resonance, and anomeric effects. An important result is that rates of alkoxide ion departure (CH3O- vs CF3CH2O-) from intermediates such as Ph(OR’,SR)CC(Ph)=NO2- and Ph(OR’,OR)CC(Ph)=NO2- show a strong dependence on the pK(a) of the respective alkoxide ion (beta(1g) close to -1), suggesting a large amount of C-O bond cleavage in the transition state.