Zinc oxide nanocrystals, pure and doped with transition metal ions, have been synthesized using colloidal techniques; after purification and concentration protocols, the ZnO solutions are mixed with monodisperse Au colloidal suspensions and used for thin film depositions. The effect of the dopant ions on the structural, morphological and optical properties of the as-synthesized colloids as well as the nanocomposite thin films has been analyzed and discussed. The dopant presence has been found to affect the CO optical sensing properties of the nanocomposite ZnO–Au films: compared to pure ZnO, an increase in sensitivity up to 80 percent and 55 percent has been detected for Co-doped and Mn-doped ZnO respectively, while Ni-doped ZnO films show only minor improvements. This observation has been ascribed to the multiple oxidation states of cobalt and manganese ions that can facilitate electron transfer between the target gas and semiconductive oxide matrix, and also to the lower surface concentration of Ni ions inside ZnO crystals, as compared to Co and Mn. A fast and reversible response after repeated CO exposures has been detected for all tested samples, and a linear response intensity with the order of magnitude of CO concentration has been observed in the 10–10,000 ppm range, with a lower detection limit of 1–2 ppm. ; peer reviewed: yes ; NRC Pub: yes