The protonation constants of ten Schiff's bases synthesized by the condensation of 2-hydroxy-1-naphthaldehyde with aniline and o-, m-, and p-substituted methylanilines, ethylanilines and methoxyanilines in various dioxan-water mixtures have been determined potentiometrically using an electrode system calibrated in concentration units of the hydrogen ion. The influence of substituents in the molecular structure on the protonation constants is discussed. The log K-2 values relating to the protonation of imine nitrogen atoms are in accordance with the Hammett relationship for all solvent compositions. A reverse relationship is observed between log K-2 for all Schiff's bases and the mole fraction of dioxan. However, the log K-1 values corresponding to the protonation of naphtholate O-ions have been found to increase with the increase in concentration of dioxan. Both electrostatic and non-electrostatic contributions influencing these constants are briefly discussed.