A series of phosphorylated flavonoids has been synthesized and evaluated in vitro for inhibitory activity against carboxylesterase, acetylcholinesterase and butyrylcholinesterase as well as for their cytotoxicity towards human adenocarcinoma A549 and human glioblastoma U251 cell lines. Diethylphosphoryl derivatives of chrysin and 7-hydroxyflavone were found to be the most effective with bimolecular rate constants for carboxylesterase inhibition k _i = 2.0 × 10 ⁶ and 5.7 × 10 ⁶ dm ³ mol ⁻¹ min ⁻¹ , respectively.