The polarographic behaviour and spectrophotometric pK(a) of 1,3-dimethyl barbituric acid and 5-phenylazo-substituted products were investigated in a buffer solution of pH 2-11. The results revealed an azo form of the substituted products; a mechanism for the electroreduction pathway was suggested. Half-wave potentials showed fair correlation with a constants while pK(a) showed almost no dependence on substituents.