As ordinarily measured, the pKa values of many substituted quinazolines and triazanaphthalenes are composite because of reversible covalent hydration of the cations. By using a rapid-reaction apparatus, the true pKa values of some of these compounds have been obtained and the equilibrium concentration ratios of hydrated to “anhydrous” cations have been calculated. The true pKa values of pteridine is predicted from the present results.