Microcystins (MC) are strongly linked to liver tumors. The degradation dynamics and characteristics of MC in an UV-microO₃ reactor with low concentration ozone generated from dry air by UV-radiation are discussed. The results showed that the photochemical degradation of MC was characterized by pseudo-first order reaction kinetics. The reaction half lives (t_1/2) for MCRR, MC-YR and MC-LR were 74.5 min, 32.2 min and 24.2 min respectively. The MC degradation wasn't affected by original concentration of MC in the water; however, it was decelerated by natural organic matter (NOM) in the water. In the UV-microO₃ process, the decrease in MC concentration was due to the destruction of the conjugated double bond on the Adda side chain by hydroxyl radicals. The UV-microO₃ process can be used for the decomposition of MC in potable water. Includes 10 references, table, figures.