Abstract: A reverse osmosis pressure vessel end cap assembly includes a seal plate positioned within a pressure vessel shell, with one or more load plates positioned to retain the seal plate in position. A load ring is captured or expanded by the load plates to secure the load ring into a channel in the interior surface of the pressure vessel shell. The end cap assembly thus provides secure yet easily removable access to the pressure vessel, capable of withstanding the high pressures of a reverse osmosis filtration system. Various exemplary embodiments of the assembly are provided.