The phase behaviour of pseudoternary systems of the type H2O-Oil-non-ionic surfactant-co-surfactant(1:4) have been investigated. The non-ionic surfactants selected are polyoxyethylated alkyl phenols and polyoxyethylated alkyl ethers with varying hydrophobicity and hydrophilicity. The phase diagram is found to consist of a smaller isotropic domain and a larger anisotropic domain. The isotropic domain grows in size with increase in both hydrophilicity and hydrophobicity of the surfactants. These results have been attributed to the solubilization of oil or water in W/O microemulsion with increase in hydrophobicity and hydrophilicity of the surfactants respectively. The percentage of water intake by the microemulsion increases with increase in emulsifier percentage with a break when the overall concentration of emulsifier is about 70-80%.