We present a comparative study of the activity and selectivity of Rh/C nanoparticles and Sn-modified Rh/C nanoparticles towards electrocatalytic nitrate reduction in sulfuric acid. Electrochemical techniques, combined with more direct analytical techniques such as mass spectrometry and ion chromatography, were applied to analyse the products obtained during the reaction. Online electrochemical mass spectrometry was employed to detect volatile products, such as nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2). The combination of online sample collection to the electrochemical cell and offline ion chromatography allows the quantitative analysis of non-volatile products, such as ammonium () and hydroxylamine (NH3OH+). Non-volatile products can be detected during nitrate reduction at Rh/C electrodes. The catalytic activity of Rh/C electrodes can be enhanced by Sn modification. N2O is the dominant volatile product at SnRh/C electrodes. is the main ionic product at the Rh/C electrodes, whereas modification by Sn also leads to the formation of NH3OH+.