The reactions of hydronium ions (H(3)O(+)) with ethanol (C(2)H(5)OH) and deuterium-labeled ethanols (CD(3)CH(2)OH and CH(3)CD(2)OH) were investigated by means of proton transfer reaction mass spectrometry (PTR-MS). Besides the protonation reaction of H(3)O(+) with C(2)H(5)OH, three fragmentation processes yielding C(2)H(5)(+), CH(2)OH(+), and H(3)O(+) ions were previously implied. In this paper, we report the branching ratios for those four channels at six different field strengths (E/N ratios) of the drift tube. The contribution of the channel that reproduces hydronium ions (H(3)O(+)) was determined by detecting H(2)DO(+) ions at m/z 20 produced in reactions of H(3)O(+) with deuterium-labeled ethanols (CD(3)CH(2)OH and CH(3)CD(2)OH). The reproduction of H(3)O(+) ions was found to be significant between 108 and 162Td of the E/N ratio, however, it did not quantitatively account for the low detection sensitivity of ethanol by PTR-MS, suggesting the presence of unidentified reaction channel(s). The unidentified reaction channel(s) might be the H(3)O(+)-reproducing channel itself, because the contribution from this channel would be underestimated in this experiment due to loss process(es) such as an H/D exchange between H(2)DO(+) and H(2)O. (C) 2009 Elsevier B.V. All rights reserved.