Measurement of the third and fourth Stokes parameters of microwave thermal emission can be degraded by nonideal radiometer characteristics. Of particular importance is the level of polarization purity and the knowledge of polarization impurity on the part of the instrument. An explicit relationship is developed between the true Stokes parameters and the measurements, as functions of nonideal polarization characteristics. In principle, knowledge of the characteristics allows the degradation to be removed by a postprocessing calibration procedure. However, errors in the knowledge are possible, resulting from inexact initial testing and/or subsequent drifting of the instrument. The effects of these errors on overall calibration are considered. It is found that there is considerably greater tolerance to the errors when incoherent detection is used (differencing ±45° linear and left- or right-hand circular polarizations), as opposed to coherent detection (direct cross correlation of vertical and horizontal linear polarizations). This is particularly true when cross-polar leakage is imbalanced. The level of nominal polarization purity that is required is also more relaxed in the incoherent case. On the other hand, coherent detection has the capability for superior performance with regard to precision.