Transformation products and toxicity patterns of microcystin-LR (MC-LR), a common cyanotoxin in freshwaters, during degradation by solar photo-Fenton process were studied in the absence and presence of two major water components, namely fulvic acid and alkalinity. The transformation products m/z 795, 835, 515/1030 and 532 can be formed through attack of OH center dot on the conjugated carbon double bonds of Adda. Transformation products with m/z 1010, 966 and 513 can be generated through the attack of OH center dot on the methoxy group of Adda. The transformation products miz 783, 508 and 1012 can be originated from the attack of OH center dot on the cyclic structure of MC-LR. Transformation products (m/z 522, 1028, 1012, 1046 and 514) formed after hydroxylation of the aromatic ring with OH center dot were also identified in this study. The toxicity study revealed that fulvic acid and alkalinity strongly influence the toxicity profiles of solar photo-Fenton treated MC-LR. Fulvic acid enhanced the detoxification whereas low level total alkalinity (1.8 mg L-1 CaCO3) inhibited the detoxification of MC-LR by solar photo-Fenton process as assessed by protein phosphatase-1 (PP-1) inhibition assay. This work provides insights on the utility of solar photo-Fenton destruction of MC-LR in water based on transformation products and toxicity data. (C) 2017 Elsevier B.V. All rights reserved.