A method is developed for the determination of the effective dissociation potential (Vd) of an alkali compound used as a seed in the low‐temperature and high‐pressure working fluid of a magnetohydrodynamic (MHD) generator. For a simple compound like potassium chloride (KCl)Vdcan be derived directly by measuring the ion density in the plasma from the current‐voltage characteristics observed in MHD generator. For complex seed compounds like potassium sulphate (K2SO4), the method leads to an effectiveVdas referred to an assumed dissociation reaction. Additional experimental information as to the specific radical involved would be required to establish the true dissociation energy. In the experiments presented here, the working gas is argon at atmospheric pressure and the temperature ranges from 2000° to 3000°K. In the temperature range used and for KCl undergoing the reaction KCl&rlarr2;K+Cl, the actual dissociation potentialVdis found to be 3.6±0.4 eV. For K2SO4the assumed dissociation reaction, K2SO4&rlarr2;2K+S+4O leads to the effectiveVd=10.7±3.1 eV.