Surfactants formulated into oil-based mud emulsifiers can adsorb to pore surfaces causing changes in rock wettability which are widely believed to impair formation permeability when drilling fluid invades the near-wellbore region. This paper describes experimental work undertaken to assess the significance of this mechanism of formation damage caused by oil-based mud emulsifiers commonly used in the North Sea. Wettability alteration, relative permeability reduction to oil (at connate water saturation) and extent of surfactant retention have been measured for a range of initially water wet outcrop sandstones treated with solutions of a commercial emulsifier. The study was then extended to consider the impact of formation damage from actual mud filtrates prepared from 80/ 20 oil based muds. Changes in wettability and relative permeability were measured for mud miltrates containing increasing amounts of commercial emulsifier. Finally, the effect of substituting commercial emulsifier with pure grade surfactant on sandstone wettability and permeability was investigated. Fired outcrop cores were employed in the experiments to evaluate specifically the effect of surfactant adsorption to pore surfaces. The potentially damaging impact of wettability alteration in causing fines mobilization is therefore not within the scope of the present paper. It is concluded from these studies that formation damage caused simply by adsorption of surfactants typically formulated into oil-based muds may be of only minor significance. Although a reasonable correlation between wettability alteration and relative permeability reduction was found, there is evidence to suggest that the damage caused by the oil-based mud emulsifiers was not attributable to wettability effects alone. Pure grade surfactant was found to induce strong changes in sandstone wettability but cause only minor permeability impairment, compared with equivalent amounts of commercial emulsifier. Precipitation of organic material (wax) derived from the commercial emulsifier was identified as contributing significantly to the damage observed.