AbstractA number of alkyl and cycloalkyl methylphosphonofluoridothionates were prepared by treatment of methylphosphonothioic difluoride with a mixture of one mole equivalent each of the appropriate alcohol and triethylamine in ether or benzene medium. Reaction velocities of the irreversible inhibition of acetylcholinesterase and butyrylcholinesterase were determined and compared with those of the corresponding phosphonofluoridates. A convenient method for the preparation of methylphosphonothioic difluoride is given.