We present the effect of thermal treatments on the transduction behaviors of HCl-doped polyaniline (PANI) nanofibers integrated in conductometric devices upon exposure to 1% H2 (carried by N2). After drying in N2 at 25ºC for 12 hours, devices showed a ~10% decrease in electrical resistance upon exposure to 1% H2. However, devices subject to 12-hour drying in N2 at 25ºC followed by further thermal treatments in N2 at 100ºC, 164ºC or 200ºC for 30 minutes showed different transduction behaviors. Specifically, devices subject to thermal treatments at 100ºC and 164ºC showed a decrease in electrical resistance by ~7% and <0.5%, respectively. More interestingly, the device subject to thermal treatment at 200ºC showed a transduction behavior with opposite polarity, i.e. a ~5% increase in electrical resistance upon exposure to 1% H2. SEM, FTIR and TGA were employed to investigate the effect of thermal treatments on the morphology and chemical characteristics of HCl-doped polyaniline nanofibers. The results indicated that the change in the devices? interesting transduction behaviors might be related to the thermal treatment effects on the HCl-doped PANI nanofibers in (i) removal of adsorbed water, and (ii) crosslinking and/or degradation of polymer backbones.