Highly doped indium-tin oxide films exhibit resistivities rho as low as 1.2 x 10(-4) Omega cm, while for ZnO films resistivities in the range of 2 to 4 x 10(-4) Omega cm. are reported. This difference is unexpected, if ionized impurity scattering would be dominant for carrier concentrations above 10(20) cm(-3). By comparing the dependences of the effective Hall mobility on the carrier concentration of ZnO and ITO it is found that grain barriers limit the carrier mobility in ZnO for carrier concentrations as high as 2 x 10(20) cm(-3), independently, if the films were grown on amorphous or single crystalline substrates. Depending on the deposition method, grain barrier trap densities between 10(12) and 3 x 10(13) cm(-2) were estimated for ZnO layers. Also, crystallographic defects seem to reduce the mobility for highly doped ZnO films. On the other hand, for ITO films such an influence of the grain barriers was not observed down to carrier concentrations of about 10(18) cm(-3). Thus the grain barrier trap densities of ZnO and ITO are significantly different, which seems to be connected with the defect chemistry of the twos oxides and especially with the piezoelectricity of zinc oxide. (C) 2007 Published by Elsevier B.V.