Tin oxide is one of the most important semiconducting materials for gas-sensing applications. The use of nanocrystalline particle films in gas-sensing devices significantly enhances the sensitivity of the devices due to the larger specific surface area and changes in the electrical properties. To understand the gas-sensing mechanism and to determine the influence of nanocrystallinity on the gas-sensing behaviour, it is important to carry out detailed electrical characterisation of the semiconductor material having well defined nanoparticle film structures. For this investigation, tin oxide (SnOx) nanoparticles were prepared using a gas phase synthesis set-up with SnO powder as the initial material. This set-up allowed the deposition of size-selected nanoparticles in the range of 10 to 35 nm and having a geometric standard deviation of sigma < 1.1, to form a nanoparticle film. To obtain information about the morphology, crystallographic structure and stoichiometry of the nanoparticles and nanoparticle films the well-known characterisation methods TEM, AES, STM, XRD and RBS were used. An automated measurement set-up was designed and fabricated for the measurement of the gas sensor characteristics and electrical parameters of nanostructured SnOx thin layers in controlled gas environments in a temperature range from room temperature to 300 °C. The set-up consisted of a gas environment chamber, a specially designed microhotplate and microhotplate holder together with a control, supply and electrical measurement equipment. Sensitivity and dynamic behaviour measurements were carried out by observing changes in the electrical conductance on exposure of gases, such as CO, CH4, C2H5OH, H2 and NO diluted in synthetic air. A systematic study was made to examine the material characteristics (particle size and chemical composition) as influenced by the process parameters of the synthesis set-up and the gas-sensing properties of SnOx layers. For the first time, it has been demonstrated conclusively that a decreasing particle size leads to an increased sensitivity and decreased response time. The effect is especially evident in the case of layers with particle size smaller than 20 nm. In addition, in-situ oxidised and post-annealed tin oxide nanoparticle films have higher sensitivity values and the maximum sensitivity shifts to lower operating temperature. The results of this work lead to a better understanding and improvement of tailored tin oxide gas sensors using monodisperse nanoparticles.