The resistivity and thermoelectric power of beta -FeSi2 single crystals and polycrystalline beta -FeSi2+x thin films doped with 3d transition metals, have been investigated for dependence on the deviation from strict stoichiometry and on the purity of the source material. It has been found that in single crystals, the thermoelectric power is characterized by a large phonon drag effect which is more pronounced in samples grown with the source material of higher purity, and by large absolute values of > 500 muV/K in a broad temperature range. The values of the resistivity prepared at the Si-rich and Fe-rich phase boundaries do not show any correlation with the deviation from strict stoichiometry, but its temperature dependence rho (T) is controlled by native defects. Polycrystalline beta -FeSi2+x thin films doped with Co and Cr, and prepared by electron beam evaporation and magnetron sputtering, were found to have maximum power factors outside the homogeneity region at approximately x = 0.15 +/- 0.05. Their thermoelectric power remains below that of single crystals.