Inelastic incoherent neutron scattering from goethite (alpha-FeOOH) and lepidocrocite (gamma-FeOOH) has been investigated using a time-focused crystal analyser (TFXA). In each case the neutron spectrum is dominated by two very sharp peaks at ca. 800 and ca. 1100 cm(-1), corresponding to out-of-plane and in-plane internal modes of the hydrogen bond. The peaks per inverse centimeter are for cc at 887 (6.0) and 1000 (4.5) and for gamma at 780 (3.8) and 1000 (5.8); intensities in arbitrary units are shown in parentheses. The peak frequencies are compared with infrared data. Scanning electron microscopy shows goethite to be composed of very small needle-Like crystallites of ca. 500 nm by 120 nm, and lepidocrocite consists of micrometer size aggregates of acicular crystals of ca. 100 nm by 20 nm.