A lanthanide borosilicate glass, also known as LaBS glass, is being considered for the disposition of surplus weapons plutonium and plutonium residues. The LaBS glass, based upon Löffler-type glasses, is chemically durable and can dissolve substantial amounts of plutonium as well as the neutron absorbers gadolinium and hafnium [1]. Samples of a prototype LaBS glass composition containing 10 wt. % plutonium were subjected to water vapor at 200 °C for periods of 14 to 56 days. The Argonne Vapor Hydration Test Procedure was followed [2]. This test, while not designed to replicate specific conditions that may be found in a potential geologic repository (e.g., Yucca Mountain), has been shown to accelerate alteration phase formation that is usually observed in low-temperature tests over extended time periods [2]. The surfaces of the glass samples, along with alteration phases, were examined with a transmission electron microscope (TEM) to determine the characteristic alteration products.