Some sub solidus relations in systems of åkermanite gehlenite compositions in the range 200° C - 700° C have been studied. Åkermanite was reported to decompose to wollastonite and monticellite at approximately 700° C (Harker and Tuttle, 1956). At 6 kbar water pressure and 500° C, wollastonite and clinopyroxene form from åkermanite. At the same pressure and ca. 450 °C. clinopyroxene and xonotlite seem to be the stable mineral association. The decomposition temperature of the åkermanite-gehlenite mixed crystal series at 4.8 bar to 6.7 kbar depends upon the composition of the mixed crystal and is situated between 500° C and 650° C. The decomposition products are vesuvianite and hydrogarnet, hydrogarnet, or hydrogarnet and xonotlite in addition to clinopyroxene and wollastonite formed from the åkermanite-rich batches. The hydrogarnets formed during the experiments are mixed crystals in the series hydrogrossular - hydropyrope.