The millimeter-wave spectra of the unstable halofulminates BrCNO arid ClCNO were recorded at room temperature in several frequency intervals between 52 and 230 GHz. Besides rotational transitions in the vibrational ground state, transitions in numerous thermally, excited states of the XCN bending mode (X = Br or Cl) could be observed. The irregular sequence of these satellites indicated that in both molecules, the XCN bending mode is highly anharmonic. Indeed, the XCNO molecules exhibit truly quasilinear behavior. From semirigid bender analyses of the rotational data, the effective potential functions and their barriers to linearity were determined. The barrier heights were found to be 130.82 (56) cm(-1) for BrCNO and 166.86 (84) cm(-1) for ClCNO, resulting in quasilinearity parameters gamma (0) of +0.362 and +0.416, respectively.