Two series of 3,6-bis(arylazo)pyrazolo [1,5-a]pyrimidine-5,7-diones were prepared, and their acid dissociation constant pK a s were determined and correlated with the Hammett equation. The results of these correlations, together with the spectroscopic data, indicated that the title compounds exist predominantly in the 3-arylazo-6arylhydrazonopyrazolo[1,5-a]pyrimidine-5,7-dione tautomeric form, both in the solid and solution phases.