The electro-oxidation of formic acid on platinum electrodes has been on the spotlight for the last decade since it presents an apparent simple, yet intricate mechanism with three known reaction pathways. Despite the formation of CO, a strongly adsorbed intermediate, it was reported an interesting behavior when hydrazine is added in this solution. The simultaneous oxidation of formic acid and hydrazine was argued to be additive, meaning that both molecules would be oxidized without one interfering to the other. However, recently, it was suggested that this process would proceed synergistically. In this work, we aim at understanding how both molecules are oxidized on a platinum surface by monitoring the formation of gaseous products with Differential Electrochemical Mass Spectrometry (DEMS). The chosen pH was acidic (similar to 0.3) and the working electrode employed was a platinum sputtered Teflon membrane. The results showed that the signal of the ratio mass/charge 44 (related to CO2 production) for the formic acid and hydrazine system exhibited a new peak at lower potentials (c. a. 0.55 V), where the oxidation of formic acid would not proceed in the absence of hydrazine. The enhanced production of CO2 in the presence of hydrazine, which does not contain any carbon on its structure, yields the spectrometric proof that a more-than-additive mechanism takes place when oxidizing the mixture. These results open a new perspective for the study of the mechanism of the formic acid oxidation and to the development of new mixed fuels for energy conversion devices. (C) 2017 The Electrochemical Society. All rights reserved.