The electric dipole moments of two weakly bound cyclic trimers, (H2O)(2)HCl and (H2O)(2)HBr, have been determined from Stark effect measurements in rotational spectra recorded at conditions of supersonic expansion. The experimental results an compared with ab initio and induction calculations and the dipole moments of all three monomers are found to be subject to appreciable enhancement on complexation. The effect on the total dipole moment of each trimer is, however, less marked owing to partial cancellation of the various component moments. A novel electrode system for Stark effect measurements in a cavity Fourier transform microwave spectrometer is also described.