Summary: This paper is concerned with the property of simple substitution, introduced by K. Sasaki [Bull. Sect. Log., Pol. Acad. Sci. 18, 94–99 (1989; Zbl 0697.03011)]; this property is related to axiomatizations of superintuitionistic propositional logics, i.e., extensions of intuitionistic propositional logic. Axiomatizations of these logics are propositional calculi containing intuitionistic propositional calculus as a subsystem; they are called superintuitionistic propositional calculi.