The role of ethylene oxide solubility in the ethoxylation processes has been studied for what concerns : the influence on the kinetics of the reaction and the behaviour in the different kinds of normally employed reactors, in particular the influence on the reactor instability. The conclusions of the work are : (i) Henry’s law, is a rough and unreliable model to describe ethylene oxide solubilities in a large range of pressures and temperatures, (ii) the predictive UNIFAC model can be applied to a few number of situations, (iii) the employment of NRTL (non random two liquids theory) and Wilson’s models seem to be more reliable, but many experimental data are necessary to evaluate the correct parameters of the mentioned models. As it will be seen it is difficult to collect and interpret experimental, data. Nevertheless, the precision in the evaluation of ethylene oxide solubilities is fundamental for reactor design, simulation and safety.