A generalized material balance equation was presented at the Escuela de Petroleos de la Universidad Industrial de Santander for coal seam gas reservoirs based on Walsh 's method, who worked in an analogous approach for oil and gas conventional reservoirs (Walsh, 1 995). Our equation was based on twelve similar assumptions itemized by Walsh for his generalized expression for conventional reservoirs, it was started from the same volume balance consideration and was finally reorganized like Walsh (1 994) did. Because it is not expressed in terms of traditional (P/Z) plots, as proposed by King (1 990), it allows to perform a lot of quantitative and qualitative analyses. It was also demonstrated that the existent equations are only particular cases of the generalized expression evaluated under certain restrictions. This equation is applicable to coal seam gas reservoirs in saturated, equilibrium and undersaturated conditions, and to any type of coalbeds without restriction on especial values of the constant diffusion.