The request of calibration of humidity sensors at high temperature and high relative humidity is quite common in many industrial sectors (food, pharmaceutical, etc.) and in environmental monitoring applications. However, in spite of a widespread traceability need in such measurement conditions, only few calibrations or industrial laboratories are available to meet this demand. This is probably due to the high costs associated with the calibration facilities and to the technical difficulties for realisation, use and maintenance of such a laboratory for humidity sensor calibration at high temperature. In the work the authors highlight the main issues connected with the realisation and characterisation of such calibration systems. Experimental investigations on the behaviour of the calibration reference standards and the traditional working instruments in the high temperature and high humidity regimes are reported. Specific calibration problems associated with the humidity sensor response curves and their dependence from the temperature are discussed. Finally, the calibration uncertainty of humidity sensors in the above-mentioned conditions is estimated.