Differential vapor pressures were measured for mixtures of two cyclic polymethylene ester oligomers in p-dioxane and chloroform at 25, 30, 35, and 40 degrees C at five different concentrations ranging from 1 to 20 wt %. The Flory-Huggins interaction parameter (chi) as well as Leonard's interaction parameter (chi') for flexible and semiflexible rings were calculated and compared to one another. A new method for the estimation of the number of segments of a cyclic polymer is proposed that allows Leonard's equations to be applied correctly to a particular cyclic compound. Consistent differences between chi and chi' were observed for all studied mixtures, and the differences became smaller if the cyclic oligomers were considered semiflexible. Interestingly, the enthalpic parameter (kappa) deduced from values of chi and chi' did not differ within their uncertainties. This supports the prediction that mixing cyclic polymer compared to its linear counterpart is mainly due to a molecular configurational entropy difference and that this difference should become less pronounced as the cyclic compound becomes larger.