A growing interest in diamond materials has been shown in the recent year for the design of smart chemical and biochemical sensors due to the remarkable physical and chemical properties of diamond. In this paper, modified diamond nanoparticles (DNPs) coatings are investigated as sensitive layers on surface acoustic wave sensor (SAW sensor) for the detection of volatile chemicals. DNPs are deposited onto SAW transducers by a layer-by-layer deposition method and then surface treated to fix them on the substrate and to enhance their affinity to specific compounds such as nitro-aromatic compounds, nerve-agent stimulants, or toxic gases. Homogeneous and reproducible coatings were achieved. The diamond coatings’ surface was either oxidised or reduced to see the effect on the response to ammonia gas, ethanol, DNT or DMMP vapours exposures. The sensors were generally very sensitive to the target chemicals and the response fully reversible. Oxidation of the surface promoted hydrogen-bond formation and therefore enhanced the response to most vapours under test. Even though the sensors were not very selective, we demonstrated the suitability of DNP coatings as stable and reliable sensing interface. This opens up wide opportunities for immobilizing more selective and highly sensitive chemical/biochemical receptors onto SAW transducer surfaces via strong covalent binding of those receptors on diamond nanoparticles deposited homogeneously on the SAW sensors surfaces.