An apparatus has been developed to react samples with hydrogen at pressures of up to 689.5 MPa (6805 atm) and temperatures of up to 200/sup 0/C. It is being used to determine whether RT/sub 5/ and RT/sub 3/ compounds (where R is a lanthanide element and T is a 3-d transition element, usually cobalt or nickel) ''fill up'' with hydrogen when subjected to high hydrogen pressures. Preliminary experiments with LaCo/sub 5/ in this apparatus showed a new (third) plateau at 126.7 MPa (1250 atm) in the PCT diagram; only two plateaus have previously been reported. There is an abrupt upturn at an H/LaCo/sub 5/ ratio of approximately 8.23. A second run showed an upturn at 8.76. The maximum loading is nine atoms of hydrogen.