Doxycycline hydrochloride has been shown to form charge transfer (CT) complexes with a number of electron acceptors in 50% (v/v) aqueous ethanol medium. CT absorption bands of complexes with four acceptors of known electron affinity have been determined, and they correlate well with the electron affinities of the acceptors. From this correlation, the electron affinity of riboflavin has been estimated to be 1.02 eV. The CT absorption bands have also been utilized to determine the vertical ionization potential of doxycycline hydrochloride (7.05 eV). The enthalpy and entropy of formation of the complex between riboflavin and doxycycline hydrochloride have been determined by estimating the formation constant (K) spectrophotometrically at five different temperatures in pure water. The pronounced effect of the dielectric constant of the medium on the magnitude of K has been observed by determining K in aqueous ethanol mixtures of varying compositions. This has been rationalized in terms of ionic dissociation of the hydrochloride DH+Cl-, hydrolysis of the cation DH+, and complexation of the free base D with riboflavin.