Oxygen and helium gases, often used in many plasma processes, were added to argon-based glow plasmas, produced at the atmospheric pressure, in order to study the controllability of the plasma characteristics by the supply gas mixing. Based on the electrical and optical diagnostics, the plasma parameters, such as the breakdown voltage, the rotational temperature, and the plasma uniformity, and their changes due to the gas mixing were investigated. The experimental results showed that the helium gas addition reduced the breakdown voltage (from 430 V to 300 V), the rotational temperature (from 465 K to 360 K), and the plasma uniformity. On the other hand, a small amount of oxygen gas increased the breakdown voltage (from 435 V to 463 V) and the rotational temperature (from 520 K to 600 K) due to various energy loss channels of the oxygen gas. The experimental results showed that it was possible to control the plasma characteristics by the gas mixing. Crown Copyright (c) 2006 Published by Elsevier B.V All rights reserved.