Over the past two years I have carried out high-pressure surveys of the ammonia monohydrate (AMH) phase diagram using a Paris-Edinburgh press mounted on the D20 beam-line at the ILL. In the course of this work I have identified four crystalline polymorphs with as-yet unknown structures and an uncertain relationship to phases observed by Richard Nelmes' group in the 1990s. The D20 data do not provide adequate resolution for indexing of these high-P powder patterns, and the objective of this proposal is to collect such data. The four new polymorphs will be reproduced and high(er) resolution powder data will be collected, which I anticipate will lead to successful indexing, and possibly also to structure solution.