A radiative-transfer model is used to assess the potential of ground-based, upwardlooking, passive microwave and millimeterwave radiometry from 5 GHz to 300 GHz for the standoff detection of chemical agents. The microwave and millimeterwave spectra of the agents and simulants were modeled using published ab initio quantum-mechanical predictions of the spectroscopic constants validated by laboratory spectroscopic measurements. The pressure-broadened microwave-to-millimeterwave rotational band contours vary in intensity, shape, and peak frequency with agent or simulant, providing the potential for both identification and quantification. The radiative-transfer calculations show that the detection sensitivity is greatest for zenith angles near vertical (θ zenith ≈ 0°), with changes in brightness temperature upon introduction of a 1 km thick, 1 µmol/mol concentration chemical agent cloud of a much as 12 K for soman for a water-vapor column of 1 cm (relative humidity ≈ 38 %). The detection sensitivity as measured by the magnitude of the change in brightness temperature decreases with increasing water-vapor column and with increasing zenith angle till it effectively vanishes at horizontal viewing angles (θ zenith ≈ 90°). The analysis determines that the performance of a realistic sensor will be limited by the atmosphere opacity near horizontal viewing angles of greatest interest for standoff detection and by background brightness temperature fluctuations due to water-column variations driven by atmospheric turbulence of short and long-term duration. It is concluded that ground-based, passive, microwave and millimeterwave radiometry is not a viable technology for the sensitive standoff detection and identification of chemical agents.