The Cl-35(-) NMR chemical shift and line width and the H-1 chemical shifts of cetylpyridinium chloride, CPyCl, change abruptly at the critical micelle concentration, indicating conversion of monomeric surfactant into micelles within a very small range of concentration. The simple pseudophase treatment fits these results up to 0.05 M CPyCl, but there then appears to be a modest change in micellar structure. Premicelles of single chain surfactants, detected kinetically or photochemically, are probably formed by interactions between reactant(s) and surfactant. (C) 2004 Elsevier Inc. All rights reserved.