Phase equilibria and critical phenomena in the ternary system potassium nitrate – water – methyldiethylamine, where a constituent binary liquid system is characterized by phase separation with a lower critical solution temperature (LCST), were studied by means of the visual-polythermal method within 25–55°C. It is established that introduction of potassium nitrate into the system water-methyldiethylamine leads to lowering of its LCST from 49.9 down to 29.1°C and to reduction of the mutual solubility of components. The distribution coefficients of methyldiethylamine between the aqueous and organic phases of the monotactic equilibrium were calculated for a number of temperatures. It is shown that the effect of sailing-out of methyldiethylamine from its aqueous solutions by potassium nitrate increases with an increase of temperature. The constructed isotherms of the phase states of the system have confirmed the generalized scheme of topological transformation of the phase diagrams of ternary systems salt-binary solvent with salting-out.