A simple extension of geothermobarometric technique is developed, from the usual case of assumed equilibrium among all minerals, to layered coronas with gradients of chemical potential. For reactions among end-members in the minerals, instead of the assumption of zero affinity (-δG) = 0, (-δG) ratios are used. The values of these ratios are defined by diffusion modelling of corona growth. The variations in composition of solid-solution minerals across the corona must be considered. The method is demonstrated using a corona, for which diffusion modelling has been published, in a metabasic granulite from the Yenisey Ridge, Siberia. Provided that an appropriate set of affinity ratios is selected, the result is fairly well constrained, varying only slightly with the choice of activity model for garnet, and indicating T = 740 ± 20°C with P = 9.5±0.7 kbar. The method indicates the extent of departure from equilibrium P,T conditions during such a reaction.