A new equation of state is developed to correlate the osmotic pressures of a n-dodecyl hexaoxyethylene ether (C12E6)-water micelle system at several temperatures and n-dodecyl pentaoxyethylene ether (C12E5)-decane-water and n-dodecyl pentaoxyethylene ether (C12E5)-sodium dodecyl sulfate (SDS)-decane-water microemulsion systems at 25.5 degrees C. The osmotic pressures for the above systems at other conditions are predicted from the obtained size parameters. The correlated and predicted results are satisfactory. The osmotic pressures of the uncharged droplets are calculated with the contributions of the surface bending energy from the deformation of droplets based on the modified Carnahan-Starling expression and van der Waals attraction. The osmotic pressures for the weakly charged droplets are calculated with the above contributions and the contribution of long-range electrostatic interactions based on the mean-spherical approximation. The adjustable parameters in the model have obvious physical meaning.