Phase relations on the join CaTiSiO 5 -CaSi 2 O 5 were determined at 1350 degrees C over the pressure range 3.5-12 GPa by a combination of synthesis and reversal experiments in a piston cylinder and a multi-anvil press. Titanite-like phases were recovered from all experiments in this pressure range. At 3.5 GPa the maximum Si VI content of titanite is 3.0+ or -0.6 mol%, whereas bulk compositions with higher Si content yield a mixture of titanite solid solution plus coesite and walstromite-structured CaSiO 3 . The maximum Si VI content of the titanite increases with pressure to 21+ or -2 mol% at 7 GPa and 46+ or -2 mol% at 7.5 GPa. At pressures of 8.5 to 12 GPa all intermediate compositions yield a single titanite phase. X-ray and TEM analysis show that these have the A2/a symmetry of the titanite aristotype. The variations of the room-pressure unit-cell parameters of the A2/a phases with composition can be described by the equations a [Aa] = 7.040(9)-0.492(15) X(Si VI ); b [Aa] = 8.713(7)-0.316(11) X(Si VI ); c [Aa] = 6.564(4)-0.220(7) X(Si VI ); beta [ degrees ] = 113.721(6) - 0.537(12) X 2 (Si VI ); V (cell) [Aa 3 ] = 367.8(9)-47.5(1.6) X(Si VI ). For the CaSi 2 O 5 composition the recovered material has I1 symmetry but is known to transform back to A2/a titanite structure at 0.2 GPa at room temperature. Similarly, with increasing pressure the P21/a CaTiSiO 5 transforms to A2/a symmetry at 3.6 GPa at room temperature. The conclusion is that at 1350 degrees C and pressures in excess of 8.5 GPa there is complete solid solution between CaTiSiO 5 and CaSi 2 O 5 based upon the isovalent exchange of Si for Ti in the octahedral site of the A2/a structure. Rietveld structure analysis of intermediate compositions reveals no evidence for ordering or intermediate phases. Preliminary experiments at pressures between 13.5 GPa and 16 GPa yielded mixtures of titanite solid solution plus perovskite and stishovite. From these data and information on the phase relations for the CaSiO 3 -CaTiO 3 join the topology of the phase relations between approximately 3 and approximately 13 GPa in the central part of the CaO-TiO 2 -SiO 2 ternary have been deduced.