A method is described that classifies water in a cellulosic fiber and water system. Thermogravitmetric analysis (TGA) is used to determine hard-to-remove (HR) water from an isothermal drying curve. The HR water content is defined as the moisture ratio (g of water / g of oven dried sample) of the fiber-water system at the transition between the constant rate zone and falling rate zone of an isothermal drying curve. The TGA instrument provides tightly controlled drying conditions that allow one to distinguish small differences in drying behavior. The exact value of the HR water content was found to be influenced by the initial moisture ratio, solid mass, and isothermal drying temperature. Experiments at optimal conditions showed that the HR water content is linearly correlated with the water retention value. This correlation was examined in terms of the similar states of the fibers (i.e. minimum saturation point) when the HR water content and water retention value are measured. The dependence of the HR water content on the solid mass of the sample revealed two constants, y-intercept and slope. The y-intercept is considered to be linked with instrumentation and the slope is to be associated with the fiber. Once the HR water content measurements are adjusted for these constants, experiments may be conducted at any solid mass down to a few milligrams. The dependence of the HR water content on the isothermal drying temperature was linked to differences in the drying rate of the constant rate zone. It is proposed that the higher drying rate for higher isothermal temperatures results in the transport of water internal to the fiber becoming an important factor at higher moisture content. This results in a higher HR water content being observed. It was also found that the pulp yield affected the HR water content. At the same water retention value, the highest HR water content was found for mechanical pulp followed by unbleached chemical pulp, and bleached chemical pulp. These differences may be attributed to differences in chemistry, pore size and volume, and the dynamics of pore collapse during drying. This method can be performed on extremely small samples and provides a convenient and insightful characterization technique for cellulosic fibers.