Alpha-cyclodextrin (alpha-CD) powder has been used to adsorb organic compounds for various applications. In order to study the complexation of alpha-CD powder with guest compounds, an understanding of physical properties alpha-CD powder including molecular structure and water adsorption behavior is essential. Two known types of alpha-CD powder structure are crystalline and amorphous. However, commercial alpha-CD powder exists only in crystalline form and there is no reported information on properties of amorphous alpha-CD powder. This study aims at characterization and comparison of those properties of commercial (crystalline structure) and spray-dried alpha-CD powders. The results of SEM, X-ray, FTIR, DSC, PSD, TGA and C-13 solid-state NMR analyses indicated that spray-dried alpha-CD powder was in an amorphous form with spherical shape particles (mean diameter similar to 13.28 mu m) and its glass transition temperature (T-g) was about 83.4 degrees C. The spray-dried alpha-CD powder crystallized when kept above 65.30% Equilibrium Relative Humidity (ERH), corresponding to 13.70-14.97 g of moisture 100 g(-1) of dry solids. In moisture adsorption study, at the same ERH levels, amorphous alpha-CD powder adsorbed moisture much higher than crystalline alpha-CD powder did, and its adsorption isotherm data were fitted well with both BET and GAB models at a low RH range (<65.30% ERH). A poor fitting to a wider RH range (ERH = 11.12-97.30%) of the GAB model was due to the crystallization of amorphous alpha-CD powders equilibrated at above 65% RH. (C) 2015 Elsevier B.V. All rights reserved.