Hydrogen cyanide (HCN) is used in industry on a wide scalefor the general benefit of mankind, e.g. various manufacturingprocesses (synthesis of nitriles, resins, dyes, chelating agents, pig-ments and fibres), electroplating and precious metal extraction.But HCN is extremely dangerous to human beings as it inhibitsthe consumption of oxygen by body tissues via breath, inges-tion, skin absorption, and ocular routes. Therefore it is extremelyimportant and highly desired to detect any possible presence orleakage of HCN [1–8]. Since Sauerbrey found experimental ver-ification of the mass-frequency shift relation for quartz crystalresonators, quartz crystal microbalance (QCM) has been widelyexploited in the field of chemical and biological sensors becauseof its many advantages such as intrinsic high sensitivity, low cost,easy installation and inherent ability to monitor analytes in realtime [9–18]. In our previous works, the application of nanostruc-tured CuO as sensing medium on QCM resonators seems to be verypromising for detection of HCN because of their attractive sensing performances (excellent selectivity, high sensitivity, good repro-ducibility and stability) [19–21]. In practical applications, however,the environmental humidity is a key factor that would signifi-cantly influence the outcome of detection because of the physicaladsorption and desorption of water molecules [18]. Therefore, itis extremely important to investigate the sensing performance ofCuO functionalized QCM resonators to HCN under varied humidityconditions.In the current work, we synthesized CuO nanoparticles by afacile solvothermal method, from which CuO nanoparticles func-tionalized QCM resonators were fabricated and explored for HCNsensing. And the sensing performance of CuO nanoparticles func-tionalized QCM resonators toward HCN was also studied in detailunder varied relative humidity conditions.