In this letter, we comment on the applicability of the derived characteristic equation (Eq. (7)) in a recently published article of Guoxin [Rev. Sci. Instrum. 86, 014704 (2015)]. To validate our comment, we first derive another characteristic function for determination of complex permittivity of dielectric materials for the configurations considered in the above article using calibration-independent uncorrected S-parameters for transmission-line measurements (coaxial-line, waveguide, free-space, etc). Unlike the characteristic equation in this article, the characteristic equation derived here for determination of the complex permittivity of liquid samples does not require any knowledge about the complex permittivity of plugs, used for holding liquid samples in place. We then performed 3-D full-wave simulations for the measurement configurations presented in Guoxin's article for substantiation of the characteristic equation derived in this letter.