Sensing electrodes were fabricated by chemical deposition of gold onto a Nafion membrane. These electrodes, with suitable choice of real surface area, applied potential, supporting electrolyte, and gas flow rate, showed a detection limit of 40 ppb ethylene for a signal-to-noise ratio of 3. A linear response was observed up to at least 500 ppm. The choice of flow rate was especially influential as it was found to affect the mechanism of the sensor response, which is kinetically controlled even at relatively low flow rates. No cross-sensitivity was found to ethanol and acetaldehyde which might be present in certain applications of the sensor, such as in horticulture.