Abstract: Continuing our investigations (poster1, 2) concerning the irreversible inhibition of serine-hydrolases (acetylcholinesterase, chymotrypsin) by organophosphates [1][2] we have prepared the racemic 3-fluoro-2,4-dioxa-3l5-phosphabicyclo[4.4.0]decan-3-ones (±)-7-(±)-12. Being conformationally restricted, these cis- and trans-decalinetype congeners with the F-substituent in the axial and equatorial position fit differently into the active site of acetylcholinesterase (AChE) as represented by their kass-values.