Acetone and ammonia are the gases in human breath with relatively high concentrations of 1–200 ppm and 0.5–2 ppm respectively. Gas sensors with vertical current channels covered with organic polymer semiconductors have been proved to have high sensitivity and reliability in breath ammonia detection. But the polymers have a large current drift in air and poor sensitivity to acetone. Here, the polymer is replaced by the iridium metal complex tris[2-(p-tolyl)pyridine]iridium(III), (Ir(mppy)3), a common emitter in organic light-emitting diode (OLED). The vertical gas sensor now has good sensitivity to acetone in the important range of 0.5–100 ppm. The current of Ir(mppy)3 sensor has a smaller drift in air during the measurement. In 800 s, the current drift of Ir(mppy)3 device is only 3.2% percent, whereas the drift of PTB7 polymer device is 35% and P3HT polymer device is 29%. The good resolution of 0.5 ppm acetone is related to the stability of the current of Ir(mppy)3 device.