We have been involved in the research and development on H2 gas sensors utilizing a high proton affinity of the pyridyl rings integrated in organic pigments such as diketopyrrolopyrroles, peryleneimides, and copperphthalocyanes. Our sensors exhibit a remarkable increase in electrical conductivity by three orders of magnitude even under 0.01% H2. However, an anomalous phenomenon has been observed in the title compounds (pyridylimidazoperylene and pyridylimidazonaphthalene) that shows an additional increase in conductivity (i.e., reverse of the sensor current) as soon as H2 shuts off. The unusual effect has been investigated in the present investigation on the basis of the crystal structure, molecular orbital calculations, and UV-visible/IR spectra with special attention to the sign of the charge carriers before and after exposure to H2. As a result, the pyridylimidazole ring is found to be responsible for the unusual effect that involves two protonation sites. In addition, the charge carriers have been determined to be solely electrons in the build up of the sensor current; whereas both electrons and protons are involved in the decay process. The contribution of the proton conduction in the decay process prevails over the electron current, ending up with the reverse in sensor current.