Time‐dependent electron density measurements of electron beam pumped XeCl laser mixtures (nonlasing) are performed using a CO2(10.6 &mgr;m) quadrature interferometer. The electron beam pulse length is ≊500 ns (full width at half‐maximum) and delivers ≳10 A/cm2at the foil. Electron densities are measured as a function of the halogen (HCl) and xenon concentration. For a 99.3% Ne/1.5% Xe/0.16% HCl mixture at 3000 Torr, the electron density peaks near the beginning of the pulse at ≊7×1014cm−3, decreases to 3×1014cm−3, then gradually increases to ≊6×1014cm−3at the end of the pulse. As the initial halogen concentration is reduced, the electron density increases, rising dramatically near the end of the pulse. This increase in the electron density later in the pulse is accompanied by a decrease of the XeCl* sidelight fluorescence.