A detailed study of the intercalative polymerization reaction of aniline with FeOCl, an extensive physicochemical and spectroscopic characterization of the products, and their oxidative behavior in air is presented. FeOCl reacts with excess aniline in CH3CN at 25 °C, in air, to form black microcrystalline products containing polyaniline (PANI) of the general formula (PANI)xFeOCl (I). X-ray diffraction from a single crystal of (PANI)xFeOCl shows that the polymer is intercalated in an ordered fashion that is commensurate with the FeOCl lattice. This ordering causes a doubling of the unit cell in the two in-plane directions (a- and c-) forming a 2a × 2c superlattice. The PANI can be extracted from (I) by FeOCl framework dissolution in acid. Molecular weight studies of extracted PANI via gel permeation chromatography (GPC) analysis suggest Mw~ 6100 and Mn ~ 3500 versus Mw ~ 69 000 and Mn ~ 7700 observed for bulk PANI, Chain lengths of PANI in the intralamellar space are shorter than those of bulk PANI but show narrower length distribution. Variable temperature 2H-wide-line NMR of (PANI)xFeOCl shows that the polymer chains are significantly confined with respect to phenyl ring rotation, but the spectra are influenced by the electron paramagnetism of the host. The (PANI)xFeOCl was also characterized by infrared spectroscopy, Mossbauer spectroscopy, electron paramagnetic resonance spectroscopy, thermal gravimetric analysis, magnetic susceptibility measurements, and charge-transport measurements. Mossbauer spectroscopy suggests a mixed-valent FeOCl with a Fe2+/Fe3+ ratio in the material ~1:9. Aerial oxidation of (PANI)xFeOCl yields an intimate mixture of PANI and β-FeOOH. The PANI after oxidation shows a molecular weight intermediate between that extracted from fresh (PANI)xFeOCl and bulk PANI. The electrical conductivity of single crystals is 1.5 × 10–2 S/cm and features a thermally activated temperature dependence consistent with a semiconductor. Thermoelectric power measurements show a positive Seebeck coefficient (~10 μV/K) at room temperature which increases steadily with decreasing temperature, reaching the value of 100–400 μV/K at 150 K, consistent with a p-type semiconductor. The charge transport data suggest that in (PANI)xFeOCl the reduced FeOCl framework dominates the electronic conduction.