We present accurate measurements of oil and surfactant self-diffusion coefficients made in the microemulsion phases of the pentaethylene glycol dodecyl ether (C12E5)-water-decane system, using the "Pulsed Gradient Simulated-Echo" NMR technique Microstructures of three microemulsion phases obtained with different surfactant-to-oil weight ratios have been analysed. Particular attention was focused on the phase boundaries where the microemulsions consist of normal oil swollen micelles in equilibrium with excess oil. Increasing the temperature, a micellar growth was supported by self-diffusion results and finally a bicontinuos phase was found.