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Stem diameter variations relative to atmospheric vapour pressure deficits and the corresponding differences in pre-dawn and late dusk foliar organic acids were measured in 4 evergreen and 2 partly drought deciduous succulent shrubs. These methods were compared at two arid South African sites in distinguishing CAM and C3 photosynthetic modes. Changes in stem diameter were continuously recorded at 2 h intervals using linear variable differential transducers, known as stem diameter variation sensors. Annual data collected over varying climatic conditions revealed that rapid changes in the stem diameter in response to vapour pressure deficit concurred with typical foliar acid trends expected for CAM and C3 photosynthetic modes. Nocturnal foliar acidification, which typifies the CAM mode, was predominant in evergreen shrubs Cephalophyllum framesii, Ruschia bolusiae, Ruschia stricta and Zygophyllum cordifolium whilst C3 mode was prevalent in drought deciduous Lycium cinereum and Tripteris sinuata. Diurnal trends in foliar acidification concurred with the predicted patterns in stem diameter variations for CAM and C3 modes, thus confirming the potential application of stem diameter sensors in distinguishing CAM and C3 modes in arid environments. Daily amplitudes in stem diameter were significantly reduced by nocturnal fog and dew precipitation in a C3–CAM shrub. This supports earlier studies that the CAM mode in facultative and CAM-cycling plants does not only conserve water, but may also aid in water absorption. Stem microvariation changes revealed CAM–C3 variations that were concealed when measuring CAM activity from total foliar acids. This technique may be very important considering that even the dependable carbon isotope technique under-estimates the number of CAM-equipped plants in natural systems.