The effect of the specific adsorption of iodide on the optical second harmonic (SH) signal generated at the surface of a mercury film electrode has been investigated. The overall SH response depends on both the metal surface charge and the iodide coverage. The latter contribution is responsible for deviations from the case of purely non-adsorbing anions like fluoride anions where the SH signal is a parabolic function of the metal surface charge. In particular, the effect of specific adsorption on the SH signal intensity is seen as an overall increase in the SH signal intensity where specific adsorption occurs. The potential corresponding to the minimum of the SH intensity parabolic curve follows the shift of the point of zero charge with the bulk solution iodide concentration, although with a small offset, the magnitude of which depends on the geometrical configuration of the experiment. For positive metal charges, the effect of specific adsorption is non-linear with the bulk solution iodide concentration. A strong enhancement of the SH intensity is indeed observed at iodide concentrations below 10(-4) M. This enhancement is attributed to the specific interaction between the mercury electronic density and the iodide anions and more specifically to the discrete nature of the adsorbed charge.