  The role of systematic errors induced by thermal fluctuations is analyzed for
the SPOrt experiment with the aim at estimating their impact on the measurement
of the Cosmic Microwave Background Polarization (CMBP). The transfer functions
of the antenna devices from temperature to data fluctuations are computed, by
writing them in terms of both instrument and thermal environment parameters. In
addition, the corresponding contamination maps are estimated, along with their
polarized power spectra, for different behaviours of the instabilities. The
result is that thermal effects are at a negligible level even for fluctuations
correlated with the Sun illumination provided their frequency $f_{tf}$ is
larger than that of the Sun illumination ($f_{day}$) by a factor $f_{tf} /
f_{day} > 30$, which defines a requirement for the statistical properties of
the temperature behaviour as well. The analysis with actual SPOrt operative
parameters shows that the instrument is only weakly sensitive to temperature
instabilities, the main contribution coming from the cryogenic stage. The
contamination on the E-mode spectrum does not significantly pollute the CMBP
signal and no specific data cleaning seems to be needed.
