Mixtures of sphingomyelin and Triton X-100 were dispersed in aqueous media and molecular details of the mixed micelles were studied by proton magnetic resonance (1H NMR). Most signals narrowed when the molar ratio of Triton X-100 to sphingomyelin increased, most likely as a consequence of looser molecular packing within the mixed micelle. These changes in molecular packing might be a prime cause for the increased rates of enzymatic hydrolysis of sphingomyelin, observed upon addition of Triton X-100. The changes in the line widths of the various constituents support the model suggested by S. Yedgar et al. [(1974a) Biochim. Biophys. Acta 363, 98-111] for the TR-SM mixed micelles. When heated to 65 °C, aqueous dispersions of TR become turbid; this temperature is defined as the “cloud point” of the detergent (Tc). Addition of sphingomyelin decreased the cloud point temperature of the mixed dispersion. Studies of optical properties and 1H NMR spectroscopy suggested that the clouding is a consequence of formation of clusters of mixed micelles of TR and SM. The motional state of the TR molecules in those micelles which are directed toward the bulk aqueous phase does not change as a consequence of the clustering. Kinetic studies were done on the effect of temperature on the rate of hydrolysis of mixed micelles of TR and SM by rat brain sphingomyelinase. Above the cloud point, the effective concentration of substrate, which is available for interaction with the enzyme, decreased, resulting in lower reaction rates at subsaturating concentrations of substrate.