The increase of the fractional micellar ionization, alpha, of sodium dodecyl sulfate, SDS, on addition of 1-butanol, BuOH, has been estimated conductimetrically. Calculations were made with the ratio of the slopes of conductivity against [SDS] above and below the critical micelle concentration, cmc, and also by application of Evans’ equation which includes the aggregation number determined by fluorescence quenching. The method of slopes gives very high values of alpha, especially at high [BuOH]. The aggregation number of 0.05 M SDS decreases from 65 in water at 25-degrees-C to 22 in 0.98 M BuOH. The cmc of SDS decreases on addition of BuOH and goes through a shallow minimum at 0.55 M BuOH. The quenching of the fluorescence of naphthalene by Br- in SDS is markedly increased by addition of BuOH and NaCl which permit incursion of Br- into the anionic micelles.