In heritage buildings, monitoring of the hygrothermal behaviour of building components and of the indoor climate is often necessary for conservation purposes. In many cases, measurement on sensitive building surfaces is required, which necessitates direct contact between the sensor and the surface over the entire measurement period. Commonly-used mounting systems cannot be removed without damage to historic surfaces. This paper describes a new system for mounting sensors on valuable historic surfaces, developed in cooperation with conservators and building physicists. Various materials and methods were tested in the laboratory and in situ for applications indoors and outdoors. Cyclododecane (CDD) was used as a protection layer to protect original surfaces, and also as a contact layer for accurate measurement. Since CDD is a volatile binding medium, sublimation must be hindered until the measurement period has ended by covering with a diffusion-tight material. CDD can be a solvent for some materials, so conservators should ensure that no reaction occurs between the covering layers, the CDD and the historic surface. The development of the covering layers and their application methods is presented in this paper. The effect of this new reversible mounting system on measurement accuracy is also examined and discussed. The system has been applied and monitored several times in museums and castles. The historic surfaces were assessed afterwards for damage or residues. The system is almost completely reversible and does not affect measurement accuracy significantly.