This study investigated the capability of vacuum UV to reduce the concentration of cyanobacterial toxin microcystin-LR (MC-LR) using low-pressure Hg lamps emitting 185 nm and 254 nm light. A collimated beam setup was used to irradiate samples of MC-LR solutions prepared in Milli-Q® water. The impact of competing water compounds was tested using solutions containing dissolved organic carbon (DOC), alkalinity (NaHCO3), and chloride (NaCl). Results showed that MC-LR in pure water at typical concentrations found in cyanobacterial bloom waters (17 and 40 μg/L) could be reduced below detection limits (0.5 μg/L) within one minute of irradiation time by a UV dose less than 40 mJ/cm2. A solution with a much higher initial concentration of MC-LR (870 μg/L) did show a reduced degradation rate. The presence of competing compounds does appear to reduce observed MC-LR degradation rates with the greatest impact caused by DOC followed by alkalinity followed by chloride. MC-LR degradation appears to occur by both direct photolysis by 254 nm photons and by advanced oxidation by hydroxyl radicals generated from 185 nm photons. Vacuum UV has shown promising capability at reducing MC-LR concentration.