The fluid encapsulation technique allows for simultaneousinsitupressure and temperature calibration, at present up to 6 GPa and 800 °C, using on‐line data processing. Each experimental run is done quasi‐isobarically with continuous variation of temperature. Temperature is determined by the thermal emf’s ofS‐ orK‐type thermocouples placed in contact with accepted standard calibration materials which undergo pressure and temperature dependent phase transitions. In the course of this study, the phase diagrams of Bi, Hg, Sn, Tl, Pb, and Fe have been reinvestigated and self‐consistency between transitions of all of these materials was used to determine thep,T(emf) coordinations. Thermal emf’s, for theS‐type thermocouple, are converted to temperature using a 20‐degree polynomial that fits the IPTS‐68 data to within 0.4 K and is continuously differentiable in the temperature range from 0 to 1705 °C.