The ionisation constants of 2-nitrobenzidine, 2,2'-dinitrobenzidine and 2,3'-dinitrobenzidine were determined spectrophotometrically in 33-3% (w/w) methanol. The low pK value obtained for 2,3'-dinitrobenzidine when compared to that of 2-nitro and 2,2'-dinitrobenzidines is explained on the basis of electron withdrawing nature of the nitrogroup and intramolecular hydrogen bonding. © 1982 Indian Academy of Sciences.