Abstract A brine 5.0 molal in NaCl and 0.45 molal in CaCl 2 was forced through random-fabric clay cakes, 1.0 to 1.6 cm thick, prepared from the 0.25–2.7 μm diameter size fraction of Cheto montmorillonite from Chambers, Arizona. Runs were made at 20°, 95°, 140° and 180°C using a compaction pressure of 34.5 MPa (5000 psi), an upstream fluid pressure of 22.8 MPa (3300 psi), and a downstream fluid pressure of 8.96 MPa (1300 psi). An additional run was made at 140°C, a compaction pressure of 68.9 MPa (10,000 psi), an upstream fluid pressure of 39.0 MPa (5650 psi), and a downstream fluid pressure of 25.3 MPa (3650 psi). At constant fluid flow rate, the molal filtration efficiencies of both Na + and Ca 2+ increase with temperature, the latter more rapidly. The effect of compaction pressure is shown by comparing the two runs at 140°C: the filtration efficiency for Ca 2+ at the higher compaction pressure was twice that at the lower compaction pressure; for Na +, it was 60% greater. At the slower flow rates found in nature, it appears that Ca 2+ must be retained preferentially relative to Na + in virtually all sedimentary environments, with the Ca 2+ preference increasing with increasing temperature. The apparent membrane enrichment for the heavy oxygen isotopes of throughput water molecules observed at 140°C is 0.15%. and that at 180°C is 0.05%., values that are opposite in sign to those reported in the literature for 20°C. This apparent shift in enrichment with temperature is 4 to 5 times the room-temperature salinity correction, but the reliability of the shift value is reduced by the possibility in the high-temperature runs of exchange between brine and clay, which would also make the brine isotopically heavier. Observed changes in filtration efficiency of Ca 2+ and Na + with temperature and fluid velocity can be explained qualitatively using arguments involving fluid viscosity and hydraulic drag. The newly-designed membrane press used in the study performed adequately at temperatures up to 180°C, but failed in several respects at 220°C.