We report the results of local density functional theory calculations on a Brønsted acid site of the zeolite H-ZSM-5. We have investigated the structural and electronic properties of the site. Comparison is made between our results and existing experimental data. It is shown that structural relaxation around an acid site must be performed to obtain accurate energetics for substitution of aluminum into the zeolite framework. The effects of cluster termination are studied by comparing results obtained for both isolated clusters and clusters embedded in a Madelung field generated by the zeolite framework. The properties of the electron density distribution in the region around an acidic proton indicate that the acidic moiety may be characterized as a rather soft acid.