P. E. M. Allen et al.: An NMR Study of the Schlenck Equilibrium of tert-Butylmagnesium Bromide in TetrahydrofuranGrignard / Spectroscopy, Nuclear Magnetic Resonance / Thermodynamics A 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 the 'H resonances (low field: tBuMgBr, high field: tBu,Mg) 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. -The I3C spectrum is assigned. The CH, resonance of tBu,Mg occurs at a lower field than that of tBuMgBr. The equilibrium parameters determined from the I3C spectra over the temperature range 282-322 K agree with those determined from the 'H spectra though their precision is less.