In this work, a microemulsion system composed of propan-2-yl tetradecanoate (IPM) as oil, polyoxyethylene (20) sorbitan monooleate (Tween 80) as surfactant, propan-1-ol as cosurfactant, and water as the hydrophilic phase has been prepared. The pseudoternary phase diagram of {IPM (1) + [Tween 80 (21) + propan-l-ol (22)] (2) + water (3)} at 303.15 K has been constructed at a constant surfactant to cosurfactant mass ratio of 0.5:1. The phase diagram shows the area of microemulsion. Densities and viscosities were measured in the microemulsion area along three water dilution lines. A maximum viscosity is observed along the water dilution line, which indicates a structural transition from water-in-oil, to a bicontinuous state, and then inversion, to the state of oil-in-water.