By using α-cyanostilbenes as Lewis acids, Lewis acidity scales have been established in ethanolic and methanolic dimethyl sulfoxide containing the appropriate 0.01 Msodium alkoxide and in the system sodium methoxide – methanol. This scale, designated, describes the ability of the solvent to add an alkoxide ion to an alkene. Thevalues range from 11.73 in methanol to 21.74 in 93.27 mole % dimethyl sulfoxide in ethanol.The most acidic indicator used to establish the scale was α-cyano-2,4-dinitrostilbene, with a pKAof 12.73 in dimethyl sulfoxide – methanol. The least acidic was α-cyano-3-trifluoro-methylstilbene, with a pKAof 21.98 in dimethyl sulfoxide – ethanol.The increase in the system's ability to add alkoxide ion to the cyanostilbenes as the dimethyl sulfoxide concentration rises is ascribed chiefly to its effect on the activity of the alkoxide ion. The functionsH−and, unlike their analogues in acid systems (H0andHr), do not diverge greatly as the concentration of the hydroxylic solvent component is decreased.