Possibilities for enhancing the performance and selectivity of malonic ester monoalkylation by optimization of phase-transfer catalysis conditions were studied. Two-phase catalyst systems containing the organic phase and a solid base are shown to be more effective in comparison with the systems consisting of the organic phase and aqueous solutions of deprotonating agents. The performance of systems composed of the organic phase and an aqueous base solution can be increased by stepwise introduction of a deprotonating agent into the system. An effective catalytic system with an organic phase consisting of reactants and solid potassium hydroxide with small admixtures of water is suggested, which makes it possible to selectively conduct the monoalkylation reaction of malonic ester under mild conditions in the presence of polyethylene glycol.