BACKGROUND Microalgae are considered a third-generation raw material for biofuel production. This work compared the conversion of the extracted oil from the freshwater microalgae Scenedesmus rubescens via transesterification for the production of biodiesel. Considering the cost of biodiesel production, heterogeneous catalysts present a significant advantage because of their stability and reusability. Biochar and modified biochar with H2SO4 were used as catalysts. RESULTS Biochar (BC) was pyrolyzed at 850 degrees C, under limited oxygen conditions, and was activated by sulphuric acid (H-BC). The catalyst: oil mass ratios tested were from 0 to 0.7:1 for BC, and from 0 to 1:1 for H-BC. The biochar sample exhibited a strong buffer capacity in the pH range between 7 and 5.5, while the acid treated biochar was strongly acidic and could neutralize a significant amount of bases. The produced methyl esters are almost the same in all cases examined. BC samples gave better transesterification results, as far as it concerns the conversion, compared to H-BC. CONCLUSION The enhanced activity of the BC with mass used may be correlated with the basic minerals presented on the biochar surface. Raw biochar gave better conversion results (47%) during transesterification compared to the modified biochar with H2SO4 (24%) with better catalyst to molar ratio, 0.7:1 and 0.1:1, respectively. (c) 2020 Society of Chemical Industry