The transition temperatures Tc of tri-glycine sulfate and isomorphous tri-glycine selenate are raised by the application of hydrostatic pressure. Up to 2700 atmos, the dependence of Tc on pressure is linear with a slope of 2.6×10-3 degrees/atmos for the sulfate and 3.8×10-3 degrees/atmos for the selenate. The Curie-Weiss law, bC(T-Tc), is unaffected by pressure except for the shift in transition temperature. The curve of the spontaneous polarization is shifted accordingly along the temperature axis; in the vicinity of the transition point, the square of the spontaneous polarization depends linearly on temperature and pressure. The results are discussed in terms of the thermodynamic theory of ferroelectrics. © 1960 The American Physical Society.