Mg-rich diopsides on the MgrSirO.-CaMgSirOu join were synthesized from glasses or gels and subsequently annealed at atmospheric pressure between 1000' and 1350'C for varying lengths of time. A marked change in the exsolution kinetics was observed at 1325'C for Diun.t and at 1275"C for Di.rs. Below these temperatures the respective composit ions exsolve rapidly (in one day or less), and the resulting microstructure, observed by transmission electron microscopy, consiSts of closely-spaced, irregular lamellae paral lel to (001). Above 1325"C (for Di*r) and 1275"C (for Diurr) no exsolut ion was observed after equivalent or longer annealing t imes. In addit ion, no exsolut ion occurred for a composit ion of Ditt .u annealed at temperatures as low as 1000'C for 144 hours. A microstructure consisting of straight widely-spaced lamellae, also parallel to (001), was observed in long-term runs ofDiuro annealed at 1125'C for several hundred hours; however, the method of synthesis was different from that used for the other compositions and may have introduced chemical inhomogeneit ies into the start ing material. In al l cases the exsolved phase is perfect ly coherent with the host, but inverts to low pigeonite during cool ing. Because ofthe coherency between the phases, the exsolution must occur beneath the c0herent solvus, and the sharp break in the kinetics is attributed to the (001) coherent spinodal.