The protonation equilibrium of methyl esters RCOOCH3 (R = Me, Et, i-Pr, t-Bu, Ph) has been investigated in aqueous sulfuric acid at 25 C with the NMR technique. Experimental points show systematic differences from the theoretical curves which prevent an accurate determination of the protonation parameters. By analogy with the observations made on ketones, it is suggested that the formation of a hydrogen-bonded complex between the ester molecule and the hydronium ion is responsible for this behaviour, and a procedure is presented which allows for the treatment of the two simultaneous equilibria (protonation and complexation), thus obtaining pure protonation parameters. The trend of m* and pK in the series if discussed in terms of the relative stabilization of the protonated ester by internal external charge dispersal, and compared to the analogous data for ketones.