The thermodynamic ionisation constant (Ka) of 3,5-dinitrobenzoic add In water from 10° in 50° has been investigated. spectrophotometrically. The resettle are shown as a function of temperature (T) in ºK by pka=-1147.01/T 6.9381-10-4 T. The changes of free energy (Δ F), entropy (Δ H), entropy (AS) and heat capacity (Δ Cp) for the dissociation process have been caleulated from the temperature coefficient of the dissociation constant. The equations relating them thermodynamic functions with absolute temperature are also even. The'pKa' of the acid increases linearly with increases In ionic strength of the buffer solution end ma be related by the equation pKa= 2.0297 I+ 2.9255= up to an Ionic strength (I) of 0.1. The value of Ka have also been found out for venom methanol-water mixtures.