AbstractA 300 MHz proton NMR determination of the thermodynamic parameters of the Schlenk equilibrium of tert‐butylmagnesium bromide in tetrahydrofuran is reported for the temperature range 285‐−330 K. In solutions where bromide is in excess over tert‐butyl groups other equilibria intervene below 285 K. However when the Grignard stoichiometry prevails concentrations are governed by the Schlenk equilibrium and its determined parameters to at least 260 K. — The Ashby assignments of the1H resonances (low field: tBuMgBr, high field: tBu2Mg) are confirmed over the temperature range of our measurements but have been found to invert below 200‐−210 K. Resonances arising from impurities are assigned. — The13C spectrum is assigned. The CH3resonance of tBu2Mg occurs at a lower field than that of tBuMgBr. The equilibrium parameters determined from the13C spectra over the temperature range 282‐−322 K agree with those determined from the1H spectra though thei