Dwarf succulents persist in the arid Succulent Karoo despite the low-water storage capacities of their contracted leaves and stems that are inadequate for enduring severe and prolonged drought. We examined the contribution of non-rainfall moisture (fog, dew, water vapour) to the water budgets and relative abundance of two endemic dwarf succulents Agyroderma pearsonii and Cephalophyllum spissum. Non-rainfall moisture was measured with automated lysimeters containing bare quartz-gravel soils and introduced A. pearsonii and C. spissum individuals at hourly intervals spanning an 8-month wet winter to dry summer period. Total non-rainfall atmospheric moisture intercepted by the bare quartz-gravel substrate of 137·6 mm, of which water vapour adsorption contributed 56·2 mm, fog 78·2 mm and dew 3·4 mm, was virtually equivalent to the rainfall amount of 142·7 mm. Agyroderma pearsonii intercepted 228·4 mm of non-rainfall moisture of which water vapour adsorption contributed 117·1 mm, fog 104·4 mm and dew 6·9 mm. This was nearly three times the non-rainfall amount of 88·7 mm y−1 intercepted by C. spissum, of which water vapour contributed 44·3 mm, fog 41·3 mm and dew 3·1 mm. The greater quantity of non-rainfall moisture intercepted by A. pearsonii corresponded with its threefold greater leaf abundance and twofold greater canopy cover than that of C. spissum. We conclude that non-rainfall moisture, especially the absorption of atmospheric water vapour by soils and its uptake by the extensive network of superficial roots of dwarf quartz-field succulents are vital in sustaining their growth and survival and in determining their distributions and relative abundance. Copyright © 2012 John Wiley & Sons, Ltd.