The micellar composition of two binary surfactant systems, that is, sodium dodecyl sulfate (SDS)/pentaethylene glycol monodecyl ether (C10E5) and SDS/lauryl amido propyl betaine (LAPB), has been analyzed by pulsed gradient spin-echo NMR (FT-PGSE NMR). The experimental data have been compared to theoretical predictions based on the regular solution theory that takes into account the nonideal mixing of the surfactant pairs. Although good agreement between experiment and theory is observed for the SDS/C10E5 pair, some discrepancy is noted for the LAPB/SDS system, particularly at a high molar fraction of SDS.