The TOPEX/Poseidon S/C, a joint venture between NASA and CNES, was launched from Kourou, French Guyana aboard an Ariane 42P launch vehicle on 10 August 1992. The primary objective of the mission is to perform precise measurements of ocean surface topography utilizing radar altimetry from a precision circular orbit with an altitude of 1335 km and an inclination of 66.25°. The S/C consists of an Instrument Module, which houses the primary instruments and S/C support equipment, and the Multimission Modular Spacecraft Standard Bus. The thermal design of the S/C includes multi-layer insulation, fixed radiators, shielded and unshielded louvers, constant-conductance heat pipes, and thermostatically and proportionally controlled heaters.This paper assesses the performance of the TOPEX/Poseidon TCS from the start of pre-launch activities throughout the first 45 days in orbit. During this time period, the spacecraft experienced initial checkout and instrument turn-on, various orbital adjustment maneuvers, and normal operational modes over a wide range of environmental conditions. The flight temperature telemetry is compared with S/C thermal math model predictions. The ability to quickly produce accurate temperature predictions for any scenario is discussed.The S/C TCS is performing as expected with the telemetry indicating that all components are operating within their allowable temperature ranges during normal mission modes and within 5°C of the flight thermal model predictions.