Micellar ionization degrees, from conductivity measurements, and micellar aggregation numbers, from steady-state quenching fluorescence of the sodium dodecyl sulfate/poly(propylene glycol) (SDS/PPG) (MW = 425 and 1000) systems in aqueous solution, have been obtained. From the experimental results we conclude that small aggregates form on the polymer chain at a surfactant concentration cl smaller than the surfactant cmc. The aggregation numbers of these aggregates sharply increase with SDS concentration and, at a critical surfactant concentration (c(2)) higher than the SDS cmc, the aggregates become large enough to incorporate the polymer chains into the surfactant micelles, forming SDS/PPG mixed micelles in aqueous solution, that are similar to the cetyltrimethylammonium bromide (CTAB)/PPG and surfactant/medium chain alcohols mixed micelles.