A thermodynamic treatment to obtain information on the stability of microemulsions and the size of the droplets in stable microemulsions is proposed. The treatment accounts for the free energy of formation of the double layers, for the van der Waals and double layer interaction potentials among the droplets as well as for the entropy of formation of the microemulsion. Because of the difficulty in computing the number of configurations which determines the entropy of formation, lower and upper bounds are derived. An approximate equation, based on an average environment attached to each droplet, is also used to compute the entropy of formation. Conditions under which a stable emulsion can occur are established and the occurrence of a phase inversion from one type of microemulsion to the other type predicted.