A falling film of liquid desiccant in direct contact with a gas stream is formed, which allows water vapor transfer between a gas stream (air) and the desiccant, enabling dehumidification and/or humidification of air. Thin films are created in one way by a wettable layer that is in contact with a support structure and in another way directly on the support structure. The devices can be installed on an absorber (conditioner) side or a desorber (regenerator) side or both of air conditioning systems; for example, liquid desiccant air conditioning (LDAC) applications.