This paper investigates the use of expanded graphite (EG) as a thermal conductivity enhancer for packed bed reactor materials in a chemical heat storage/pump system based on magnesium hydroxide (Mg(OH)(2)) dehydration and magnesium oxide (MgO) hydration reactions. Composites of Mg(OH)(2) powder mixed with EG (hereafter termed EM composites) were developed by varying the mass mixing ratios of Mg(OH)(2) to EG (psi). EM composites and pure Mg(OH)(2) powder were compacted in slab configurations (width 20 mm x length 100 mm x variable thickness 20-38 mm) and the thermal conductivities of the slabs were measured as a function of the slab density and psi. Using EG as a thermal conductivity enhancer in the composite improved the thermal conductivity of the EM slab six-fold relative to that using pure Mg(OH)(2). Evaluation of the thermal effusivity and chemical heat storage capacity showed that EM prepared with a mass ratio of psi = 8 allowed achievement of good heat transfer performance without negatively impacting the chemical heat storage capacity.