The solubility parameter and the strictly regular or quasi-crystalline versions of the regular solution theory have been applied to the calculation of the surface tension of the noncritical interface of eight mixtures of the class dimethylsiloxane or an alkane + a perfluoroalkane near their upper critical end points using the experimental upper critical solution temperature T-UCS as the only mixture-property input. The solubility parameter version of the regular solution (SPRS) theory predicts the horizontal inflection in the critical isotherm which is both observed experimentally and was first predicted by Widom from a much more detailed treatment than that used here. In contrast to the results from the SPRS theory, the strictly regular solution theory predicts this inflection only ata temperature well below T-UCS. For the limited number of cases demonstrated here the agreement with experiment is good for oligomeric mixtures but poor for non-oligomeric mixtures, surprisingly so given their lesser complexity.