We have investigated the initiation and propagation of filiform corrosion on coated aluminum surfaces using a new experimental set-up for in situ Fourier transform infrared (FTIR) microspectroscopy and the scanning Kelvin probe. During the initiation partially hydrolyzed aluminum chloride was formed in a defect on the coated surface. The conditions in the defect resembled the environment that is found in growing pits with low pH and high chloride concentration close to the coating interface. This weakened the adhesion of the organic coating and initiated the formation of a filament. The movement .of the active head was followed with in situ FTIR microspectroscopy in humid air using the characteristic IR band around 2500 cm-1 from Al(H 2O)6 3+. This band is present in spectra from partially hydrolyzed aluminum chloride, which was found in the head during propagation of the filament. The absorption of water in the hygroscopic corrosion products in the head of the filament was followed by measuring the changes in the intensity of the Al(H2O)6 3+ band during variations in the relative humidity. The hygroscopic properties of the aluminum hydroxy chloride salts formed in the head have important consequences for filiform corrosion, especially concerning the influence of climatic parameters such as the relative humidity and the effect of wet/dry transitions on the propagation of filiform corrosion.