The Parasite Reactor is a 1-liter pressure bomb for phase studies under irradiation at temperatures and pressures up to at least 400°C and 5000 psi. Provision was made for sampling of solid, liquid, and gas phases for analysis. Power output was measured. Yields of radiolytic gases were calculated. It was established that uranyl nitrate solutions are sufficiently stable up to 248°C to be used for a proposed high-pressure Water Boiler reactor.