Abstract: The interactions of ascorbic acid (Ⅰ) with 12 excipient were studied. Samples were equilibrated, dried and then diffuse reflectance spectra (DRS) were measured. DRS showed that there were no interactions between Ⅰ and citric acid, stearic acid or butylated hydroxytoluene (BHT), and that physical adsorptions appeared between Ⅰ and starch, dextrin, talc, microcrystalline cellulose, lactose or mannitol. DRS also indicated that chemisorptions existed between Ⅰ and magnisium stearate, calcium stearate or glycine, and sometimes the color of samples changed, so they were considered not compatible with ascorbic acid. The declining of concentration of Ⅰ in equilibrated mixture were studied on Ⅰ-talc system and Ⅰ-magnisium stearate system, Data presented showed that the concentration declining of Ⅰ in the former could be neglected, and that in the latter was obvious, which indicate that there was chemisorption or significant chemical interaction in the latter system. Using the same amount of solvent, repeated washing the sample with small amount each time was better than washing once with the total amount. Washing technique can be used to distinguish the physical adsorption from the chemisorption.