The effects of isothermal heat treatment of the ZhS6U superalloy (at an operating temperature of 950°C after a standard vacuum heat treatment) on the stability of its structure and mechanical properties were investigated. It was established that long-term holding of this alloy at the operating temperature is accompanied by the formation of M6C carbides, coalescence of the secondary γ' phase, and precipitation of embrittling topologically close-packed (TCP) phases of the σ and χ types. The employment of a high-temperature heat treatment of the melt (HTTM) at the stage of ingot melting changes the morphology and volume contents of structure constituents of the alloy. The transformations in the course of long-term holding at the above temperature also occur somewhat differently. It was shown that the use of HTTM in manufacturing the alloy requires improvement of the regimes of its heat treatment.