Dissociation constants of the protonated form of imidazole base have been measured from 0 to 50° at 5° intervals. The use of different formulæ for the extrapolation of data to zero ionic strength is discussed, and the results are compared with the available literature values. The acid dissociation constant of the imidazolium ion is given by the equations pK_a= 472·269/T+ 15·1061 – 0·0487442T+ 0·00005440T₀, or pK_a= 1906·884/T+ 0·6225 – 0·0000848T; the thermodynamic quantities derived from the first equation are discussed.