We have studied friction of MoS2 single layers on mica and Al2O3 in the range of 5%-90% relative humidity. Friction coefficients have been determined simultaneously for the MoS2 platelets as well as for the support. Only a slight influence on the humidity was found for the friction coefficient of MoS2 and Al2O3, On the other hand, water adsorption on mica caused a strong increase of the friction coefficient above 40% relative humidity. The change of the friction coefficient was so strong that the contrast of the friction images inverted. At very high humidities (above 60%), the friction coefficient of mica reverted to its initial value, probably because water acted as a boundary lubricant.