<p>Microwave electrothermal thrusters enable electrodeless electrothermal propulsion with propellants beyond noble gases, but molecular propellants introduce chemistry that can compete with translational heating. We report an experimental comparison of hydrogen and ammonia in a 17.8-GHz resonant-cavity microwave electrothermal thruster at tens of watts. Because the present hardware employs a straight-orifice nozzle, chamber performance is evaluated via characteristic velocity (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(c^*\)</EquationSource> </InlineEquation>) calculated from hot-fire stagnation pressure and mass flow rate, with a nozzle-equivalent specific impulse estimate provided for scaling. Hydrogen demonstrates markedly higher chamber performance than ammonia. Ammonia, despite exhibiting a stronger stagnation-pressure increase, shows degraded characteristic velocity—evidence that absorbed power is preferentially partitioned into dissociation and internal excitation rather than equilibrated heating. Consistent with this interpretation, optical emission during ammonia operation shows NH/N₂ band features and Balmer H_<i>α</i> emission. The results highlight a key design lesson for microwave electrothermal thrusters using molecular propellants: pressure rise is not, by itself, a reliable proxy for effective chamber energy conversion.</p>