Various microemulsion formulations were evaluated as reaction medium for synthesis of a surface active compound, decyl sulfonate, from decyl bromide and sodium sulfite. The reaction rate was fast both in water-in-oil and in bicontinuous microemulsions based on nonionic surfactant. Two-phase systems with added phase transfer agent (quaternary ammonium salt or crown ether) was much less efficient. It is postulated that the low efficiency of the phase transfer agents in catalyzing the reaction is caused by strong ion pair formation between the product formed, decyl sulfonate, and the phase transfer agent. To prove this point decyl bromide was reacted with two other nucleophiles, sodium cyanide and sodium azide. Neither of these give a reaction product that can form ion pair with the phase transfer agent. With these reagents phase transfer catalysis was almost as efficient as synthesis in microemulsion. It was also demonstrated that the rate of decyl sulfonate formation in microemulsion can be increased further by addition of a small amount of cationic surfactant. The choice of surfactant counterion is decisive of the effect on reaction rate, however. Whereas a small non-polarizable ion, such as acetate, gives a considerable reaction rate increase, a large polarizable ion, such as bromide, slows down the reaction. Bromide is believed to interact so strongly with the interface that it prevents the reacting ion, sodium sulfite to reach into the interfacial zone.