In this article, we characterize the composition of different fibers from agro-industrial waste by means of wet characterization methods, such as Van Soest Wine and TAPI standards. The compositions of these fibers are also determined by thermogravimetric analysis (TGA) as an alternate form of analysis and to assess the reliability of the wet methods. The original fibers are characterized, and samples are taken at each intermediate step of the wet method. These samples are analyzed using TGA, scanning electron microscopy, and infrared spectroscopy. We discuss the results of these techniques and conclude that each of the steps of the wet characterization methods is not efficiently removing the corresponding phase. We show that the wet methods affect and modify the biomasses during the analysis, while the thermogravimetric analyses with deconvolution of signs provide an accurate determination of the different components, in shorter time periods.