The moisture sorption isotherm (MSI) is very useful for biomass drying and storage. In the present work, measured MSI data of herbaceous and agricultural biomass (HAB) were collected from the literature. The adsorption or desorption isotherms at the same temperatures (15, 20, 25, 30, 35, and 40 degrees C for adsorption and 20, 25, 30, and 35 degrees C for desorption) were compared, separately, to demonstrate the similarity, evaluate the difference in the MSI among HABs, and explore the feasibility of applying the similarity for MSI prediction. It was observed that the MSIs of HABs at the same temperature are similar, with most measurements falling within a range of +/- 20% from an average given by a Guggenheim, Anderson, and de Boer (GAB) model. The chemical composition and distinct sorption behaviors of the biomass components were identified as the major factors determining the similarity and defining the variation in the MSIs of HABs. Factors associated with the measurement, including the temperature of drying for sample preparation and uncertainty related to sample heterogeneity, may cause the variation of the MSI comparable to the range of +/- 20%. Considering the similarities among HABs, using the average to estimate the MSI of an unknown HAB has a relative error of < +/- 20%. Estimating the MSI based on the chemical composition and isotherms of the major components (cellulose, lignin, and hemicelluloses as well as the extractives) can achieve the same accuracy. Both may be acceptable for practical applications. Additionally, it was found from the comparison that HABs, on average, have evidently higher MSIs than wood species mainly as a result of their higher contents of hemicellulose and the extractives.