Figures 1-6 are the Hammett and Taft plots. The effect of substituents (X) on the pKa values of 5-X-1,2,3,4-tetrazoles is rather complex (table 1) . Tetrazole itself is a strange molecule with 80% nitrogen of the total weight of the molecule. It is known that the tetrazole molecule exists in two tautomeric forms as shown below in scheme 1 . In a nonpolar medium, both the 1H and 2H tautomers are predicted to exist in comparable amounts. In the solvents like water with high dielectric constant the existence of more polar 1H-tautomer is appreciable . From figures 1 and 2, it is clear that the correlation of pKa values with para and meta substituents is good (R = 0.9769 and 0.9931) with a deviation of NH2 group with the para-substituents correlation. Though the content of 2H-tautomer is less than the 1H-tautomer in more polar solvents (since the pKa values are from water as the solvent ), yet the para and meta substituent effects originate from less polar and less abundant 2H-tautomer only. If one looks at the 2Htautomer (scheme 1), the functional group (N2-H) is at meta to X at position 5 via nitrogen 1 and it will be para to X at position 5 via nitrogens 3 and 4. This is just similar to the visual observation for defining the para and meta positions of 5 membered heterocycles as given in scheme 2 and scheme 3 as shown below.