A molecular beam probe technique has been used to investigate the role of ion bombardment of the film surface during the plasma polymeriz-ation process.Thin films have been deposited in a vacuum chamber from a molecular beam containing all the plasma species and these were compared with similar deposits from a beam with the charged particles deflected away.A comparison was made of deposition rates, vis/UV and IR absorption spectra of the films.It was found that the deposition rate was increased 20% with the elimination of ion bombardment.Ions incident on the film surface during deposition cause a net ablation of the film.The ion bombardment also causes an increased absorption of UV and near UV light by the film resulting in a noticeable yellowing.No change was observed in the IR spectra of the films with ion deflection.