An experimental investigation has been conducted to determine the frequency stability of a pulsed, rf‐excited waveguide CO2laser. Unlike transversely excited atmospheric CO2lasers, for which the optical extraction of energy (laser‐induced medium perturbation) is typically the dominant source of frequency instability, the driver in the long pulse (>30 &mgr;s) waveguide laser is the thermal heating of the gain medium due to the rf input energy. Modeling of the chirp in the pulsed waveguide laser is performed. Included are both finiteV‐Teffects in the short term and heat conduction and particle migration out of the laser bore by the gaseous gain medium in the long term.