A review of past and current work with mixed crystals of the KH,PO, family is presented. It concentrates on solid solutions where the cations have been mixed in large proportions to achieve properties different from those of the parent crystals. A large part of the review is devoted to mixed crystals of RbH,PO, and NH4H,P04, or RADP, which is a model system for structural glasses that has recently received considerable attention. © 1987, Taylor & Francis Group, LLC. All rights reserved.