The protonation constants of adenine (K(1) and K(2)) and adenosine (K(2)) were determined in binary mixtures of water with methanol or ethanol containing (0, 10, 15, 20, 25, 30, 35, 40, and 45) % (v/v) using a combination of potentiometric and spectrophotometric methods at 25 degrees C and constant ionic strength (0.1 mol.dm(-3) sodium perchlorate). The protonation constants were analyzed using the normalized polarity parameter (E(T)(N)) and Kamlet, Abboud, and Taft (KAT) parameters. A very good linear correlation of log K versus the normalized polarity parameter was obtained. Dual-parameter correlation of log K versus pi* (dipolarity/polarizability) and alpha (hydrogen-bond donor acidity) as well as pi* and beta (hydrogen-bond acceptor basicity) also gives good results in various aqueous organic solvent mixtures. Finally, the results are discussed in terms of the effect of the solvent on the protonation constants.