The hydroxides Me(OH)2 of the bivalent transition metals Co, Cu, Fe, Mn, Ni, Cd and Zn play critical roles as sorbents, catalysts or reductants in many environmental and electrochemical applications. These hydroxides can form in different forms depending on the composition of their interlayer space. The compact b-form contains no intercalated guest species whereas the a-form can host water molecules and anionic species leading to an expanded basal dimension of the unit cell. Well-crystalline b-Me(OH)2 are commonly synthesised at high temperatures rendering the a-forms undiscovered or poorly characterised. This study builds on a recent discovery of a-Fe(OH)2 material studied at GEM and aims to provide complete crystallographic details and relationships between a- and b-Me(OH)2 forms, and a better understanding the dynamics and role of intercalated water in structural stability.