Abstract Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (HTs), anionic clays, and mixed-metal layered hydroxides (MMLHs), are unusual among oxide and hydroxide materials in having large permanent positive layer charges and, consequently, large anion exchange capacities. Interest in the LDH phases has increased rapidly in recent years due to their role in a wide range of natural and hazardous waste environments, in Portland cements, and their use as catalysts, carriers for drugs, and other technological applications (e.g., Cavani et al., 1991 ; Basile et al., 2001 ). LDHs can be easily synthesized with a wide range of main (hydroxide) layer cations and interlayer organic and inorganic anions (e.g., Miyata, 1975, 1983 ; Constantino et al., 1998; Newman and Jones, 1998 ; Choy et al., 1999 ). These materials thus offer an excellent opportunity to investigate fundamental structural and dynamical properties of interlayer and surface species and are important models for understanding aqueous solutions confined in nano-pores ( Kagunya, 1996 ; Kagunya et al., 1997 ; Kirkpatrick et al., 1999 ; Kalinichev et al., 2000 ; Wang et al., 2001, 2003 ; Hou et al., 2002, 2003 ; Kalinichev and Kirkpatrick, 2002 ). The positive layer charge of LDHs provides important contrast with the negative layer charges typical of aluminosilicate clays. The structures of most LDHs consist of single-layer metal hydroxide sheets alternating with interlayers that contain anions and water molecules (e.g., Taylor, 1973 ; Bellotto et al., 1996 ; Newman and Jones, 1998 ). The hydroxide sheets develop positive charge by cation substitution, which is compensated by the interlayer and surface anions (Fig. 1 ). In many LDHs, the hydroxide layer can be thought of as a trioctahedral, brucite-type sheet of composition [M +2 (OH) 2 ] in which some of the divalent cations are replaced by higher charge cations. A typical structural formula of LDH is [M1 1−x +2 M2 x +3 (OH) 2 ]·A x/n - n · mH 2 O, where x is the fraction of trivalent cations (M2) in the structure, n is the anionic charge, and m is the number of water molecules per formula unit. The main layers of LiAl 2 LDHs, which have typical structural formulae of [LiAl 2 (OH) 6 ] · A 1/n -n · mH 2 0, can be thought of as dioctahedral gibbsite sheets with Li occupying the vacancies. Again anions occupy interlayer and surface sites to compensate the positive layer charge and are accompanied by water molecules. Understanding of the chemical behavior and transport properties of the interlayers and surfaces of LDHs and other layered structure materials requires knowledge of the molecular scale structural environments of the interlayer and surface species and their dynamical behavior on many length and time scales. Because of the diversity of structural and chemical environments in layered materials, such data also provide a broad and important basis for understanding the behavior and properties of water and aqueous fluids confined in nano-scale volumes. Traditional diffraction methods have proven insufficient to adequately address these issues, but a wide range of spectroscopic, synchrotron-based X-ray scattering, and molecular computational methods are now making significant progress in understanding interlayers and mineral-fluid interfaces (e.g., Brown et al., 1999 ).