Polyoxymethylene is used as the organic vehicle for ceramic and metal powder injection moulding. It is ideal for studies of shrinking-core reaction kinetics in porous Systems because it decomposes exclusively to monomer in the solid state in the presence of nitric acid vapor. The solid-gas reaction interface can be defined to within 3 mum by electron microscopy and about 50 mum optically. Reaction rates were measured in crowded powder assemblies containing 56 vol % alumina powder by observation of the boundary; a method that avoids the ambiguities of gravimetry. The reaction-rate constant and the effective diffusion coefficient of gas in the porous reacted layer were calculated from the shrinking core kinetics. The diffusion coefficient was in good agreement with theoretical estimates involving counterdiffusion. The time for complete depolymerization of a moulding was estimated for different powder characteristics, geometries, and volume fractions.