Organic pigments were once extensively investigated as photoconductors for the electrophotographic photoreceptor. Nowadays, they play an important role as colorants for color copies as well as materials for optical discs, electroluminescence, FET etc. Here, we report another novel application of organic pigments for H₂ gas sensors. We have previously developed a H₂ gas sensor that utilizes a high proton affinity of diketodipyridylpyrrolopyrrole. We extended our system to other pigments such as perylene-imides with pyridyl rings. Then, we encountered an anomalous effect in pyridyl-imidazoperylene(PIP) that the sensor current increases even after shut-off of H₂. In the present investigation, a similar compound called pyridylimidazonaphthalene (PIN) has been tackled in order to systematically interpret the anomalous effect in PIP and PIN. As a result, the two N atoms in the pyridylimidazo group are responsible for the effect. Furthermore, PIP is found to be quite inert to H₂O and H₂. This makes a string difference between PIN and PIP which is closely linked to the sensitivity of the H₂ sensor.