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 80 kbar (900 degrees 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 80 kbar and 900 degrees C. A sample of Li2COSiO4-P2(1)/n transforms to the denser Pmn2(1) polymorph at 40 kbar/900 degrees C, but decomposes at higher pressure (60 kbar/900 degrees C). No evidence on any novel Li2MSiO4 polymorph is detected in any of the high-pressure products. (C) 2008 Elsevier B.V. All rights reserved.