We report on a three-step, continuous-flow synthesis of [6,6]-phenyl-C₆₁-butyric acid tert-butyl ester (PCBtB), a promising electron-acceptor methanofullerene for solution-processed bulk-heterojunction solar cells, through the integrated use of three elements in a microstructured flow-reactor that eliminate the need of isolation steps, with a definite safety and environmental advantage.