Melamine and its hydrolysis products (ammeline, ammelide, and cyanuric acid) recently attracted great attention as major food contaminants. Developing analytical tools to quantify them requires exact knowledge of their acid dissociation constants (pK(a) values). Herein, we calculated the pK(a) values of these melamine derivatives in water, using a density functional theory quantum mechanical method [B3LYP/6-311++G(d,p)] in combination with the Poisson-Boltzmann continuum solvation model. The excellent agreement of the calculated values with the experimental ones shows that our method can be used to predict such properties of other food contaminants.