The effect of microemulsion having different solution structures such as O/W, W/O and bicontinuous on apparent pK values of the indicators was studied. There exist some relations between the pK values of the indicators in microemulsion and the hydrophilic character of the indicators. The microemulsions were composed of SDS, butanol, heptane and water. Indicators involved are Cresol Red, Alizarin Red S, Methyl Red, Bromothymol Blue, Bromocresol Green, and Chromazurol S.