This Bureau of Mines investigation measured the low-temperature heat capacities of feocl(c) and fecl3(c) in the temperature ranges of 6 to 305 k and 4.6 to 300 k, respectively. The standard entropies (s deg., 298.15 K) of feocl(c) and fecl3(c) were derived as 19.73+/- 0.03 Cal/deg-mole and 35.33+/-0.07 Cal/deg-mole. A large lambda transition was observed in the heat capacity of fecl3(c) at 8.4 K. The standard enthalpy of formation (ah deg. F, 298.15 K) of feocl(c) was determined to be -98.23+/-0.22 Kcal/mole by hcl solution calorimetry. High-temperature enthalpy measurements (h -h298.15) Of feocl(c) were determined to 700 k by precision drop calorimetry. Standard thermodynamic formation data and related functions were tabulated at various temperatures for feocl(c) and fecl3(c).