A new isotherm equation was developed in the framework of the vacancy solution theory. In the model, vacancies in the adsorbed phase are dealt with as a kind of reactive entity, which leads to more flexible adsorption isotherm equations. The model can be called Reactive Vacancy Solution Theory (RVST). Practical RVST equations for single-component and binary-component adsorption systems were derived by using Wilson or NRTL equations for activity coefficients. The RVST equations were found to coincide with the Langmuir-Freundlich equation when the effect of activity coefficients is assumed to be negligible. Furthermore, it was also found that the RVST model conforms to other isotherm equations when it is simplified. Validity of the RVST-W and RVST-N equations for single-component adsorption was tested. The results suggest that the highly non-ideal adsorption data are successfully correlated using the RVST models.