Spectroscopic and calorimetric measurements have been made using a 50 kW radio frequency inductively coupled plasma torch operated at atmospheric pressure with maximum temperatures and electron densities near 8500 K and 3×1021m−3, respectively. The plasma flowed through a water‐cooled quartz test section which enabled the study of nonequilibrium effects on both a recombining pure argon plasma and a recombining argon plasma with hydrogen, nitrogen, or neon. The pure argon plasma is found to be well described by a partial equilibrium model in which the free and bound‐excited electrons are in mutual equilibrium irrespective of possible departures from equilibrium with the ground state. The addition of just tenths of a percent of either atomic hydrogen or nitrogen (but not neon, in contrast) is found to significantly affect the plasma’s state of equilibrium for electron densities roughly less than 1021m−3because of a nearly gas‐kinetic reaction between argon’s first excited state and the diluent’s ground state. This ‘‘quenching’’ reaction provides a depopulating mechanism for argon’s first excited state and thereby inhibits the establishment of partial equilibrium which then invalidates several common diagnostic methods. The extent of quenching depends on the particular diluent, the amount of diluent relative to the electron number density, and on the temperature. These experimental observations are supported by an appropriately modified argon collisional–radiative model.