4-Benzenethiol-functionalized high-surface-area graphite powder was prepared and decorated with bimetallic Cu100-xRhx nanoparticles (NPs) to serve as electrocatalysts for the reduction of nitrates. In the first step, the HSAG powder was grafted with in-situ-generated diazonium compounds from 4-aminothiophenol (ATP) in an acidic medium using NaNO2 for the diazotization process. The surface composition was tuned using different initial quantities of ATP. The surface XPS-determined S/C atomic ratio was found to increase stepwise with the initial quantity of amine. In a second step, the grafted and untreated HSAG powders were decorated with Cu100-xRhx NPs by a wet chemical method and the elemental composition of the end composites was assessed by EDS-SEM and ICP, whereas TEM and EDS-TEM served to characterize the NP morphology and their composition on the nanometer scale. In all cases, the NP size was invariably found to be ∼1.7 nm but with a size distribution becoming narrower under an increasing grafting rate and the global composition enriched in copper. Voltammetry was performed with a cavity microelectrode to evaluate the electrocatalytic performance of the composites for nitrate reduction. Increasing diazonium grafting led to a progressive reduction of the peak current intensity and a shift of the peak potentials toward cathodic values. The maximum intensity was obtained for 0.005 μmol of diazonium salt per mg of HSAG, with a gain of 40% in comparison to the best untreated sample. This improvement and a change in the voltammogram characteristics after grafting seem to result from modifications of the local composition at the level of NPs that differ from the global composition. This work conclusively shows that diazonium surface modification is important not only in attaching electrocatalytic NPs to carbon supports but also in providing a narrower size distribution of the electrocatalysts together with finely tuned catalytic properties.