The construction of a high-pressure (up to 800 MPa), wide temperature range (173-600 K) differential thermal analysis (DTA) apparatus is described. Some results on the phase transitions in poly(diethylsiloxane) and poly(dipropylsiloxane) under high pressure are presented. The influence of the mesophase on phase transitions in linear polysiloxanes under high pressure, as well as the influence of pressure on mesophase formation is considered.