Critical micelle concentrations (CMCs) of several surfactants in water have been determined by conductivity measurements under hydrate-forming conditions (2 degrees C; 40 bar methane) and under the same conditions with nitrogen. Control experiments were also performed at room temperature and pressure. Surfactants investigated were the anionics sodium dodecylsulfate (SDS), sodium laurate (SL), sodium oleate (SO), 4-dodecylbenzenesulfonic acid (DBSA), and the cationics dodecylamine hydrochloride (DAHCl) and dodecyltrimethylammonium chloride (DTACl). For SO, DBSA and DTACl, CMC values were found to vary slightly under hydrate-forming conditions with respect to normal P-T, whereas SDS, SL and DAHCl solutions underwent precipitation before reaching the CMC under hydrate-forming conditions. Interestingly, no micellar formation was observed for any surfactants in the concentration range where strong hydrate promotion was previously reported. (c) 2005 Elsevier Ltd. All rights reserved.