In this paper, a numerical method for solving a higher-order model that relates the measured transmission coefficient to the permittivity of a material is used to determine the permittivity of thin grain samples. A method for resolving the phase ambiguity of the transmission coefficient is presented. Results calculated by using the higher-order model for the permittivity of grain over a range of moisture contents and frequencies are presented and compared to those calculated by using an approximate model. The advantage of using relatively thin grain samples is that a wider range of grain moisture contents can be reliably measured for a fixed sample thickness.