In this study, plasma-polymerized thin films were prepared from plasma enhanced chemical vapor deposition (PECVD) of acetylene (C 2 H 2 ), acetylene/nitrogen (C 2 H 2 /N 2 ), or acetylene/ammonia (C 2 H 2 /NH 3 ). When N 2 or NH 3 was mixed with C 2 H 2 in the feed, the films were identified to contain all elements of the mixture and the properties of the films were implied by the C–H bonds and nitrogen functionalities. As shown by X-ray photoelectron spectroscopy (XPS) the [N]/[C] atomic ratio varies by changing the mixture composition and reaches a maximum of 0.12 for mixing C 2 H 2 with NH 3 . It is found that the resistance of the thin film sensors prepared from C 2 H 2 , C 2 H 2 /N 2 , and C 2 H 2 /NH 3 is distinctly decreased by over 2 orders of magnitude by the adsorption of ethanol vapor.