The chemical composition of 5 tropical grasses, divided into stalk and leaf, at 3 maturity stages, was used to predict carbohydrate fractions by equations of the Cornell Net Carbohydrate and Protein System (CNCPS) or equations from our research group. Carbohydrate fraction A is a rapidly fermented pool that is primarily composed of sugars, some organic acids and short oligosaccharides. Fraction B1 is constituted mainly of starch and pectin. Fraction B2 has a slower rate of degradation and is available cell wall carbohydrates. The C pool is unavailable cell wall, which includes lignin. These carbohydrates fractions are estimated based on NDF analysis. However, NDF does not recover pectic substances and other ND soluble oligosaccharides such as β-glucans, fructans, galactans, etc. that are part of the cell wall matrix. Structurally, NDF is not complete plant cell wall. Then, a crude cell wall (CW) preparation, which represents plant cell wall matrix more completely because it preserves those carbohydrates that otherwise would be solubilized by the ND solution, was used in equations to predict carbohydrate fractions. By substituting NDF for CW, it was found that pectin and other ND soluble oligosaccharides (soluble fiber - SF) actually appeared in the fraction A of CNCPS, the rapidly degradable carbohydrate pool, instead of fraction B1, as proposed in the original CNCPS model. However, location of SF in the fraction A seems inadequate because degradation rate of SF is lower than fraction A components; thus, an alternative could be to place SF in a specific carbohydrate fraction (B2). This B2 fraction, soluble fiber, can be estimated by subtracting NDF from CW preparation. Because in the original CNCPS model the slowly degradable cell wall carbohydrates were assigned as fraction B2, we suggest naming this carbohydrate pool a new fraction, B3. With this arrangement, the fraction B1 would be constituted only by starch. These fractions are expressed on total carbohydrate basis, here suggested as: CHO = 100 - (CP + EE + MM + Lignin). This equation excludes lignin from the CHO compartment.