Published experimental data including garnet and clinopyroxene as run products were used to develop a new formulation of the garnet-clinopyroxene geothermometer based on 333 garnet-clinopyroxene pairs. Only experiments with graphite capsules were selected because of difficulty in estimating the Fe3+ content of clinopyroxene. For the calibration, a published subregular-solution model was adopted to express the non-ideality of garnet. The magnitude of the Fe-Mg excess interaction parameter for clinopyroxene (W-FeMg(Cpx)), and differences in enthalpy and entropy of the Fe-Mg exchange reaction were regressed from the accumulated experimental data set. As a result, a markedly negative value was obtained for the Fe-Mg excess interaction parameter of clinopyroxene (W-FeMg(Cpx) = - 3843 J mol(-1)). The pressure correction is simply treated as linear, and the difference in volume of the Fe-Mg exchange reaction was calculated from a published thermodynamic data set and fixed to be -120.72 (J kbar(-1) mol(-1)). The regressed and obtained thermometer formulation is as follows: where T = temperature, P = pressure (kbar), A = 0.5 X-grs (X-prp - X-alm - X-sps), B = 0.5 X-grs (X-prp - X-alm + X-sps), C = 0.5 (X-grs + X-sps) (X-prp - X-alm), X-prp = Mg/(Fe2+ + Mn + Mg + Ca)(Grt), X-alm = Fe/(Fe2+ + Mn + Mg + Ca)(Grt), X-sps = Mn/(Fe2+ + Mn + Mg + Ca)(Grt), X-grs = Ca/(Fe2+ + Mn + Mg + Ca)(Grt), X-Mg(Cpx) = Mg/(Al + Fe-total + Mg)(Cpx), X-Fe(Cpx) = Fe2+/(Al + Fe-total + Mg)(Cpx), K-D = (Fe2+/Mg)(Grt)/(Fe2+/Mg)(Cpx), Grt = garnet, Cpx = clinopyroxene. A test of this new formulation to the accumulated data gave results that are concordant with the experimental temperatures over the whole range of the experimental temperatures (800-1820 degrees C), with a standard deviation (1 sigma) of 74 degrees C. Previous formulations of the thermometer are inconsistent with the accumulated data set; they underestimate temperatures by about 100 degrees C at > 1300 degrees C and overestimate by 100-200 degrees C at < 1300 degrees C. In addition, they tend to overestimate temperatures for high-Ca garnet (X-grs approximate to 0.30-0.50). This new formulation has been tested against previous formulations of the thermometer by application to natural eclogites. This gave temperatures some 20-100 degrees C lower than previous formulations.