Dynamic fluorescence quenching measurements have been performed on pyrene derivatives (pyrene (Py), 1-pyrenebutanoic acid (PBA), and 1-pyrenedodecanoic acid (PDA)), using as quenchers nitroxide free radicals (2,2,6,6-tetramethyl-1,1-piperidinyloxyl, 4-hydroxy-2,2,6,6-tetramethyl-1,1-piperidinyloxyl, and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxy (TEMN)) in aqueous solutions of sodium dodecyl sulfate. The mean aggregation number values are comparable with the literature data only when the partition coefficient of the quencher is higher than 1100 M-1. It is shown that the dynamic fluorescence quenching for the PBA/TEMN pair cannot be described by the Infelta-Tachiya model owing to the fact that the intramicellar quenching rate constant is lower than the exit rate constant of the quencher from the micelle. The average location of the fluorescent probes is also discussed, Py and PDA having the pyrenyl moieties located at approximately the same depth in the micellar core, while in the case of PBA the pyrenyl moiety is buried deeper.