Elastic Recoil Detection analysis of different types of indium nitride films has been performed using a 200 MeV Au beam. Recoil ions were detected with a gas ionisation detector featuring a large detection solid angle. Severe and non-linear nitrogen depletion has been observed, with films grown by RF-sputtering losing nitrogen more quickly than MBE-grown films. Assuming the formation of molecular nitrogen as the decisive step leading to nitrogen loss, the nitrogen depletion process has been modelled using the bulk molecular recombination model. The model allows accurate extrapolations of the original nitrogen content of the material. Since the other important elements can also be quantified, the stoichiometry of the film can thus reliably be obtained from Elastic Recoil Detection analysis. All the films analysed have been found to have excess nitrogen.