A detector for the study of neutrino oscillations with a projected sensitivity to Deltam 2 of 10 Gamma3 eV 2 and to sin 2 2` of 0.1 is described. It is to be installed 800 m from three pressurized water reactors at the Palo Verde Nuclear Generating Station near Phoenix, Arizona. The detector is segmented and filled with 12 tons of gadolinium-loaded liquid scintillator. It will be placed in an underground vault with an overburden of 46 mwe and surrounded by a 1-m-thick water buffer and a hermetic active muon veto. Reactor antineutrinos are detected through proton inverse fi decay in the hydrogen-rich scintillator, and the resulting positrons are discriminated from fast neutron background by requiring a prompt coincidence across several cells between the positron and its annihilation radiation.