Thin films of 2,5-dimethyl-N,N’-dicyanoquinonediimine (DCNQI) radical ion salts are prepared by three different techniques : solid state reaction, wet chemistry, and vapour assisted reactions. The charge transfer from the metal to the organic acceptor is investigated by infrared transmission and reflection-absorption spectroscopy. The data are compared with that of powdered crystals in KBr pellets. The reaction is monitored spectroscopically in situ beginning with the deposition of the neutral films until the equilibrium state at the end of the reaction is reached. In the course of the solid state reaction we observe first the formation of crystal-like order in the neutral material and then the transfer of charge characteristic for the formation of the radicals. The films containing copper or silver require acetonitrile (CH3CN) in order to promote the redox reaction yielding the salts with the stoichiometry M(DCNQI)(2) with M = Ag, Cu. Without CH3CN, 1:1 salts are formed which obviously represent a barrier layer stopping the reaction. The temperature dependence of IR spectra and conductivity data are influenced by internal and substrate-induced stress.