A comprehensive study of the reaction scope of methyl 1,2,3-triazine-5-carboxylate (3a) with alkyl and aryl amidines is disclosed, reacting at room temperature at remarkable rates (<5 min, 0.1 M in CH3CN) nearly 10000-fold faster than that of unsubstituted 1,2,3-triazine and providing the product pyrimidines in high yields. C4 Methyl substitution of the 1,2,3-triazine (3b) had little effect on the rate of the reaction, whereas C4/C6 dimethyl substitution (3c) slowed the room-temperature reaction (<24 h, 0.25 M) but displayed an unaltered scope, providing the product pyrimidines in similarly high yields. Measured second-order rate constants of the reaction of 3a-c, the corresponding nitriles 3e and 3f, and 1,2,3-triazine itself (3d) with benzamidine and substituted derivatives quantitated the remarkable reactivity of 3a and 3e, verified the inverse electron demand nature of the reaction (Hammett ρ = -1.50 for substituted amidines, ρ = +7.9 for 5-substituted 1,2,3-triazine), and provided a quantitative measure of the impact of 4-methyl and 4,6-dimethyl substitution on the reactivity of the methyl 1,2,3-triazine-5-carboxylate and 5-cyano-1,2,3-triazine core heterocycles.