The study presents first data on the δ18O performance of poikilohydrous lichen ground cover, and its potential impact on the isotopic composition of water fluxes arising from subjacent soil layers. As a model organism, the globally distributed lichen Cladina arbuscula was studied under laboratory conditions as well as in the field. During a desiccation experiment, δ18O of the lichen’s thallus water and of its respired CO2 became enriched by ∼7‰ and followed a similar enrichment pattern to that expected from homoiohydrous, vascular plants. However, the observed degree of enrichment was lower in comparison to vascular plants due to (i) the lichen’s inherent lower evaporative resistances; and (ii) a stronger effect of the more depleted surrounding water vapour. In lichens growing in their natural habitat, this specific pattern may show substantial variations depending on prevailing microclimatic conditions.Within a field study, thallus water δ18O of lichens principally proved to become more depleted when close to equilibration with the surroundings. It thereby strongly depended on the absorption of surrounding water vapour. Moreover, the results indicate that lichen mats substantially reduce evaporation rates arising from subjacent soil layers, and may alter the isotopic signal of vapour diffusing away from these layers into more depleted values. Keywords: Craig–Gordon model; Evaporation; Lichens; Oxygen-18; Soil; Vapour; Water.