The relation between the molecular properties of a surfactant and the phase equilibria in the corresponding surfactant -water system is investigated. The emphasis is on the qualitative features that emerge from previous quantitative studies. The systems discussed are: poly(ethyleneoxide)alkyl ether -water, dialkylphosphatidylcholine -water, alkanoate -water, and alkanoate -alcohol -water. While the interactions involving the alkyl chains are essentially the same in these systems, there are large differences in the nature of the interaction between the polar groups. For each system the dominating interactions involving the polar groups are identified. For the ionic surfactant this is the direct ion -ion interaction, while for the zwitterionic phosphatidylcholines the headgroup -headgroup and headgroup -solvent interactions are both important. It is argued that the hydrophilicity of the ethyleneoxide chains is mainly due to the entropy of mixing with the aqueous solvent, while there is an effective repulsive interaction between an ethyleneoxide unit and the water above room temperature. The repulsion increases strongly with increasing temperature which has important implications for the temperature dependence of phase equilibria.