The iron oxides and iron oxyhydroxides exist as several different polymorphs, and a thermodynamic understanding of these polymorphs can provide us with an understanding of their relative stability and chemical reactivity. This study provides heat capacity measurements for lepidocrocite (gamma-FeOOH) over the temperature range (0.8 to 38) K and akaganeite (beta-FeOOH) over the range (0.7 to 302) K. Fits of the heat capacity of the two samples below T = 15 K showed similar behavior to previously published fits of goethite (alpha-FeOOH), which required a linear term and an anisotropic gap parameter to model accurately the antiferromagnetic spin-wave contributions. The akaganeite measurements were compared to previously reported measurements all of which showed significant disagreement. It is believed that the measurements reported here are the most reliable. Also, the presence of adsorbed water contributes significantly to the heat capacity of akaganeite, and the standard molar entropy at T = 298.15 K of the hydrated form was calculated to be (81.8 +/- 2)J. mol(-1) . K-1. (C) 2010 Elsevier Ltd. All rights reserved.