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This work presents the transient temperature measurement and modeling of thermochromic variable emitters using a lab-scale cryothermal vacuum test setup. A cryostat is used to provide a space-like environment with a high vacuum and an 80 K heat sink, while a custom-designed sample holder is employed to heat up the sample with transient temperature measurement. Validation with a tungsten mirror is conducted with careful calibration of heat losses as a function of sample temperature. Approaches to reduce the heat losses are discussed as well. A previously fabricated variable emitter made of thermochromic [Formula: see text] thin film in a Fabry–Perot nanophotonic structure, whose infrared emittance increases with temperature upon [Formula: see text] insulator-to-metal phase transition, is experimentally tested at different heating power inputs. A transient heat transfer model is also developed to validate the measurements, and a thermal homeostasis effect with reduced temperature swing from the variable emitter is predicted in comparison to a commonly used static emitter. This novel cryothermal vacuum test platform would facilitate the lab-scale thermal testing of novel variable-emittance coatings for space heat control applications.