Polyaniline can be inserted in V2O5·nH 2O xerogel by in situ oxidative polymerization/intercalation of aniline or anilinium in air. The reaction is facile and topotactic, forming polyaniline as the emeraldine salt. The interlayer separation (5.6 Å) is consistent with a monolayer of polymer chains in the V2O5 framework. Evidence is presented that oxygen acts as an electron acceptor both during the in situ reaction and long after intercalation is complete. The crucial role of oxygen in this reaction is probed and discussed. In an alternative route, anilinium is first intercalated and then, in a second step, is oxidatively polymerized in the xerogel upon exposure of the intercalate sample to air. Upon standing in air (aging), two processes occur independently in these materials: (a) the partial reoxidation of the reduced V 2O5 framework and (b) further oxidative coupling of anilinium and aniline oligomers inside the V2O5 layers, leading to longer chain molecules. These observations are supported by several physicochemical data. The magnetic moment of (PANI)xV 2O5·nH2O decreases gradually upon exposure to air, but it does not change when the sample is stored in vacuum. Gel permeation chromatography (GPC) analysis results show that the molecular weight of polyaniline extracted from aged (PANI)xV2O 5·nH2O is larger than that extracted from the fresh samples. The thermal stability of polyaniline extracted from aged (PANI) xV2O5·nH2O is better than that extracted from fresh samples. All (PANI)xV2O 5·nH2O samples are paramagnetic with a Curie-Weiss and a temperature-independent van Vleck contribution. Variable-temperature 2H-wide-line NMR of (PANI)xV2O 5·nH2O shows that the polymer chains are sterically confined with respect to phenyl ring rotation. The room-temperature conductivity of the freshly prepared (PANI)xV2O 5·nH2O samples is in the range 10 -4-10-1 S/cm depending on the degree of polymerization inside the layers, but the conductivity of aged samples is always greater. Room temperature thermoelectric power is negative and varies (-30 to 200 μV/K) depending on the polymer content and the degree of polymerization.