The rate of dissolution of water and dodecane in micro-emulsions (or micellar solutions) made of water/SDS + l-pentanol/dodecane has been measured using a fast mixing equipment. The dissolution rates were found to be much larger than the reciprocal of the time required to achieve an intimate mixing between the microemulsion and water (or dodecane) about 5 × 102s−1except in the water-rich region of the pseudo-ternary phase diagram where they range from below 0.1s−1to above 5 × 102s−1The inter-facial tension between oil and microemulsion appears to play only a minor role in determining the changes of dissolution rate. The results have been accounted for in terms of a dissolution process based on diffusion-controlled collisions between oil (or water) droplets and microemulsion droplets. The differences between the rates of dissolution in the water-rich region and the oil-rich region are attributed to electrostatic repulsions occurring only in the water-rich region, and which appear to play a predominant role in determining the dissolution rates of additives into oil/water microemulsions.