A Langmuir probe and a microwave interferometer have been combined to measure the electron density of argon glow discharges in the Gaseous Electronics Conference reference reactor [Bull. Am. Phys. Soc.36, 207 (1991)]. The two techniques indicate the same charge density at 100 mTorr to within 30%. This 30% difference is easily explained by the experimental peculiarities. While the predicted charge densities obtained from the two techniques track one another as the applied rf voltage is varied at constant pressure, they do not track one another as the pressure is varied at constant rf voltage. In fact, the charge densities predicted from the Langmuir probe using Laframboise’s theory (Tech. Rep. 100, Univ. Toronto Inst. Aerospace Study, 1966) are factors of 2 and 4 times lower than those from the interferometer at 250 and 500 mTorr, respectively. It appears that the probe alters the charge density in its vicinity when the probe radius becomes greater than the ion mean free path. The interferometer has also been used to investigate the macroscopic perturbation of the plasma electron density caused by the Langmuir probe. It was found that presence of the probe in a radio‐frequency discharge does not appreciably alter the electron density measured by the interferometer at a set rf voltage irrespective of the potential placed upon it. This was not the case for dc discharges in the same apparatus where the volume‐averaged electron density could be depleted by as much as 70% when the probe was biased even a few volts above plasma potential.