This paper presents a theoretical equation relating the dimensionless times of Newman's solutions of the diffusion equation for desorption and sorption. The derived equation provides a theoretical proof of the linear relationship between dimensionless time and the inverse of transport ratio for a given fraction of diffusing substance as observed numerically in the literature. Following the same approach proposed earlier by the writer, a theoretical expression for the diffusion coefficient is derived as a function of time and the inverse of desorption rate when the fraction of diffusing substance is 0.5. Evaluation of the diffusion coefficient from an experimental desorption curve is discussed.