This present work demonstrates a non-intrusive measurement of the rate constant for sodium-argon cluster ions (Na+·Ar) recombining with electrons. The measurements begin with resonance enhanced multi-photon ionization of the Na followed by coherent microwave scattering (radar) to monitor the plasma density. The Na+·Ar adduct was formed in a three-body reaction. The plasma decay due to recombination reactions was monitored as a function of time and modeled to determine the rate constant. At 473 K, the rate constant is 1.8−0.5+0.7×10−6cm3/s in an argon buffer at 100 Torr and initial Na number density of 5.5 × 1010 cm−3.