Molybdenum oxide (MoO_x) thin films were deposited by two different techniques of reactive sputtering and their properties were characterized in order to determinate their usage in gas sensors. For preparing the samples the methods of radio frequency (RF) and direct current (DC) magnetron reactive sputtering were used. The composition and microstructure of the films were studied by X-ray photoelectron spectroscopy (XPS), electron probe microanalyser (EPMA), X-ray diffraction (XRD) and Raman spectroscopy. The films' surface was observed by Scanning electron microscopy (SEM) and EPMA. The research was focused on the sensing behavior of the MoO₃ thin films. In order to do that, films of various thickness were deposited on quartz resonators and the quartz crystal microbalance (QCM) method was used. Applying it can be build high sensitive gas sensor capable to detect changes in the molecular range. These prototype QCM sensing structures with MoO_x sensitive films in as-deposited state and without heating the substrates showed good sensitivity to ammonia at room temperature. They are also being tested for sensitivity to other gases and with future development can be successfully introduced into advanced chemical sensing devices. [DOI: 10.1380/ejssnt.2009.796]