The sheet resistance, optical transmittance and microstructure of tin-doped indium oxide (ITO) thin films (50-100-nm thick) rf sputter deposited on polymer substrates are investigated using a four-point probe, spectrophotometer, X-ray diffractometer and a transmission electron microscope (TEM). Sheet resistances vary from 250 k Omega/sq. to 170 k Omega/sq. Sheet resistances for the ITO films on polycarbonate substrates are at least an order of magnitude higher than those ITO films deposited on glass substrates at the same time. Annealing ITO films on polycarbonate substrates at 100 degrees C in air for 1 h decreased the sheet resistances significantly (almost by 50%). The X-ray diffraction data indicate polycrystalline films with grain orientations predominantly along (222) and (400) directions. TEM photographs show two distinct regions of growth : a dense growth close to the substrate and a sparse growth away from the substrate. The vertical growth is columnar and rod shaped. Changes in the ITO film sheet resistance either due to the types of substrate used or due to annealing can be correlated to the grain size and grain orientation.