We investigated the inclusion of some pharmaceutically related molecules, such as l,4-benzoquinone (BQ), 9,10-anthraquinone (AQ), anthracene (AN), acridine (AC), phenothiazine (PT) and thianthrene (TH) within beta-cyclodextrin (beta-CD) using an electrochemical method as well as UV-visible spectroscopy. From the decrease in the peak current, os from the variation in the absorption spectra, the formation constants (K-f) of these inclusion complexes were obtained. The experimental results suggest that both the dimension and polarity of the guest molecule affects the inclusion complex stability. beta-CD is able to accommodate both BQ and AQ, but due to the steric factor, the two kinds of molecules penetrate into the cavity by different models, which further influences their absorption properties in different ways. AN, PT, TH and AC, whose dimensions are similar, are included by beta-CD with K-f decreasing in the order TH > AN > PT much greater than AC. These phenomena can be explained in terms of polarity, i.e. electron-accepting ability of the hybrid atoms in 9,10-positions.