Reactive extrusion has been applied as a very effective and fast process for controlled chemical degradation of a commercial polyamide 6 with trimellitic anhydride in the melt. A reaction mechanism based mainly on the amide/anhydride reaction has been discussed and confirmed by measurements of the solution viscosity, end group analysis, SEC, NMR, and IR spectroscopy. The systematic loss of molecular weight of the polyamides resulting from PA chain rupture and the formation of imide linkages and carboxyl end groups were investigated. The theoretical values of molecular weight and end group concentration of the obtained oligoamides predicted by the reaction mechanism are in good agreement with the experimental ones.