Volatile organic compounds (VOCs) in the indoor air have adverse effects on the dwellers residing in a building or a vehicle. One of these effects is called sick building syndrome (SBS). SBS refers to situations in which the users of a building develop acute health effects and discomfort depending on the time they spend inside some buildings without having any specific illness. Furthermore, monitoring volatile organic compounds could lead to early diagnosis of specific illnesses through breath analysis. Among those VOCs formaldehyde, acetaldehyde can be listed. In this thesis, VOC detecting thin film sensors have been investigated. Such sensors have been manufactured using semiconducting metal oxides, ligand activated gold nanoparticles and Graphene/TiO2 mixtures. Advanced gas deposition unit, have been used to produce NiO thin films and Au nanoparticles. DC magnetron sputtering has been used to produce InSnO and VO2 thin film sensors. Graphene/TiO2 sensors have been manufactured using doctor-blading. While presenting the results, first, material characterization details are presented for each sensor, then, gas sensing results are presented. Morphologies, crystalline structures and chemical properties have been analyzed using scanning electron microscopy, X-ray diffraction and X-ray photo electron spectroscopy. Furthermore, more detailed analyses have been performed on NiO samples using extended X-ray absorption fine structure method and N2 adsorption measurements. Gas sensing measurements were focused on monitoring formaldehyde and acetaldehyde. However, responses ethanol and methane were measured in some cases to monitor selectivity. Graphene/TiO2 samples were used to monitor NO2 and NH3. For NiO thin film sensors and Au nano particles, fluctuation enhanced gas sensing is also presented in addition to conductometric measurements.