A new method for in-situ film characterisation during thin film deposition processes was developed using waveguide mode spectroscopy. The advantages of the method are a high sensitivity for thin films in the nm-regime and a relatively simple and low-cost set-up. The detection method relies on a double grating coupler allowing a background-free detection of the propagation constants of guided optical modes. The set-up is based on a reflected mode m-line geometry requiring no moving parts. The method was applied to thin film diagnostics in plasma-assisted deposition processes and allowed for the simultaneous determination of thickness and refractive index of the plasma polymer film during deposition. The in-situ waveguide mode spectrometer was also used to monitor gas phase reactions of plasma polymerised maleic anhydride thin films on-line.