The chemisorbed water molecules form a percolation path of ionic conductivity (green dashed lines) involving the GO's functional groups acting as additional proton hopping sites. This effect results in the enhanced conductivity of the low-conductive material. (ii) Films with a higher fraction of rGO, however, exhibit a lower density of hydroxy and epoxy groups. At the same time, the restored sp 2 system imparts an enhanced baseline conductance. Commonly, rGO is described as a p-type conductor. Water acts as electron donor, interfering with the hole-dominated charge transport, resulting in an increase of resistance. At low r.h., the sorbed water molecules cannot form an extended network throughout the material. Although a certain degree of ionic conductivity may be established in these films also at lower r.h., the resulting increase of conductance is hidden by the overall higher baseline conductance and superimposed by the attenuated hole transport. Mixed sensor responses are attributed to balancing the opposing effects for intermediate degrees of reduction. (iii,iv) Finally, at high r.h. levels, secondary water layers are physisorbed, in all types of films. Hence, the conductance via the Grotthuss mechanism becomes predominant, resulting in an overall increase of conductance.