Frames for mounting windows within the walls of enclosures for isolating or sealing substances such as radioactive or other hazardous substances from the surrounding environment. In a first embodiment of the present invention, a window assembly includes a two-piece frame that is welded into the wall of an enclosure. In a second embodiment of the present invention, a window assembly includes a three-piece frame that is clamped to the wall of an enclosure. The frame assemblies may also be used to mount structures other than windows to enclosure walls, such as ducts or pipes intended for passing power, gas, vacuum, or other utilities into the interior of an enclosure.