Kyanite eclogite in the Central Swiss Alps, partially hydrated during decompression, shows conspicuous domains containing unusual phases, such as staurolite, corundum, and hercynite-dominant spinel. Detailed analysis of these domains indicates that they formed by prograde growth of porphyroblasts of lawsonite, kyanite, and garnet, prior to the complete conversion of igneous plagioclase to omphacite. In the evolution following the culmination in pressure, the chemically differentiated domains remained largely distinct from the matrix. Quantitative thermodynamic models set up for each domain allow us to analyze each domain type, in order to test (a) to what extent they behaved as closed systems, (b) whether local equilibrium was maintained during decompression, and (c) what reaction mechanisms and volumes were involved in the observed transformations. Results indicate that metasomatic effects were quite limited; local equilibration was possible to mid-crustal levels at temperatures near 700°C; reaction paths followed involved minimal transport of Al. Both prograde and retrograde parts of the P‐T path could be deduced for eclogite samples from the Southern Steep Belt of the Central Swiss Alps. A late-orogenic, prograde thermal overprint (heating spike) is confirmed.