In order to determine the phase diagram of the symmetric van der Waals model for gas-liquid binary systems we employ a method developed by Kincaid and Cohen for the molecular held model of the metamagnet. The method is based on an expansion in the dominant order parameter. The similarities and difference of the magnetic system and the van der Waals equation are discussed. It is shown how their method can be simplified and adapted to more general cases. The result is compared with the calculations of Korteweg and found to be in accordance with his results.