Abstract Hydrogen peroxide (H 2O 2) is one of the most stable reactive oxygen species (ROS), which is produced in plants in various metabolic processes and its level can increase as a result of many stress factors. It was found that a NaCl-induced shift from C 3 to CAM in Mesembryanthemum crystallinum L. leaves was accompanied with a significant increase in H 2O 2 concentration (as calculated on a chlorophyll basis). Moreover, the highest accumulation of H 2O 2 in leaves and leaf veins appeared before the maximal nocturnal malate accumulation was detected on day 6 and 9 of NaCl-treatment, indicating that salinity itself is responsible for an increase in H 2O 2 production. Several symptoms of functional CAM, i.e. strong increase in NADP-ME activity, and diel malate oscillations, appeared after 9–12 days of salt-treatment. The obtained results suggest that accumulation of H 2O 2 in leaves and leaf veins is due to salinity itself, and appears before functional CAM, but H 2O 2 as a trigger involved in C 3-CAM transition seems to be an important factor.