  The instrumental polarization induced by a flat isotropic dielectric slab in
microwave frequencies is faced. We find that, in spite of its isotropic nature,
such a dielectric can produce spurious polarization either by transmitting
incoming anisotropic diffuse radiation or emitting when it is thermally
inhomogeneous. We present evaluations of instrumental polarization generated by
materials usually adopted in Radioastronomy, by using the Mueller matrix
formalism. As an application, results for different slabs in front of a 32 GHz
receiver are discussed. Such results are based on measurements of their complex
dielectric constant. We evaluate that a 0.33 cm thick Teflon slab introduces
negligible spurious polarization ($< 2.6 \times 10^{-5}$ in transmission and $<
6 \times 10^{-7}$ in emission), even minimizing the leakage ($< 10^{-8}$ from
$Q$ to $U$ Stokes parameters, and viceversa) and the depolarization ($\sim 1.3
\times 10^{-3}$).
