AbstractDissociation constants of 3‐ and 4‐pyridineboronic acid and σ‐values for themeta‐ andpara‐B(OH)2and ‐B(OH)3−groups have been determined; in aqueous solutions the compounds occur predominantly in the pyridiniumboronate form. 2‐Pyridineboronate seems to be thermally unstable, whereas 3‐ and 4‐pyridineboronic acids are thermally stable but decompose upon treatment with methyl iodide.Irradiation in neutral or slightly basic solution of 3‐ and 4‐pyridineboronic acids effects splitting into borate and pyridine (in its turn photochemically converted into “photopyridine”).We suggest that these thermal and photoreactions may be considered as SEl‐protodeboronations of the pyridinium boronate form. Experimental reactivities could be roughly correlated with results of P.P.P. calculations of the change of electrostatic energy betwee