The adsorption behavior of sodium dodecyl benzenesulfonate (NaDDBS) on a raw (as received) coal sample and its demineralized variety with 11.3% and 1.2% ash contents respectively has been studied. The samples have been characterized by their proximate analysis, particle size distribution, surface area, porosity, density, points of zero charge, etc. Adsorption of NaDDBS on these two samples has been studied as a function of concentration of NaDDBS, temperature, pH, and presence of indifferent electrolyte in the medium. It has been observed that the isotherm exhibits two adsorption plateaus below and above the critical micelle concentration (CMC) of NaDDBS. Low heats of adsorption suggest weak hydrophobic bonding between adsorbent and adsorbate. The present work aims to correlate the adsorption of surfactant onto coal particles with the theological behavior of coal-water slurry (CWS). The results reveal that addition of a very small amount of NaDDBS (0.3 wt% of coal) to 60% (w/w) CWS results in a marked reduction of the apparent viscosity of the CWS at a shear rate of 100 s(-1). The effect of pH on the apparent viscosity of CWS with and without the presence of the surfactant is also investigated. (C) 2003 Elsevier Inc. All rights reserved.