The aggregation of alkyltrimethylammonium bromide (C(n)TAB) surfactants in very dilute aqueous solutions of sodium polyvinylsulfate (NaPVS) and sodium dextransulfate (NaDxS) has been investigated. The aggregation number, N-s, for the surfactant in complexes with the polyions was estimated with time-resolved fluorescence quenching. For C(12)TAB-NaPVS the aggregates at 50 degrees C are considerably larger (N-s = 120) than free micelles, in contrast to what was found earlier (Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115) for complexes with sodium polyacrylate. The interaction with NaDxS also results in large micelles (N-s = 107). Binding isotherms for C(12)TAB in 0.5 mM NaPVS solutions at 25 and 50 degrees C were determined. The effect of temperature is small. The observed high cooperativity in the binding is consistent with the large aggregation number. The effect of NaDxS concentration on the cac was investigated and found to be equivalent to the effect of simple salt. The relation between intrinsic binding strength, cooperativity, and aggregation number is discussed.