The behavior of Li3PO4 and Li2MSiO4 (M =Mn, Co) compounds under high pressure/high temperature is investigated. Pmn2 1 -Li3PO4 remains stable up to the higher experimental limit of 80kbar (900°C). A sample of Li2MnSiO4 consisting of a mixture of Pn2 1 and Pmnb polymorphs converts upon high pressure/high temperature treatment into the Pmn2 1 polymorph; the latter being stable at 80kbar and 900°C. A sample of Li2CoSiO4-P2 1 /n transforms to the denser Pmn2 1 polymorph at 40kbar/900°C, but decomposes at higher pressure (60kbar/900°C). No evidence on any novel Li2MSiO4 polymorph is detected in any of the high-pressure products.