The long‐term stability of epitaxial thin film structures of superconducting Y1Ba2Cu3O7−xon silicon wafers (100), using a yttria‐stabilized ZrO2(YSZ) buffer, is presented and compared to identical structures on SrTiO3(100) and yttria‐stabilized ZrO2(100) single crystals. For Y1Ba2Cu3O7−y/YSZ/Si heterostructures, the maximum Y1Ba2Cu3O7−yfilm thickness is limited to 50 nm; otherwise thermal strain induces microcracks. Thinner films are more stable, but nevertheless show aging over several weeks, which affects critical current density and room‐temperature resistivity, but not the critical temperatureTc.