Frequency- and voltage-dependent charge pumping characteristics of thin film transistors (TFTs) are discussed. Methods for identifying interface trap and oxide charge parameters based on these characteristics are presented. Correlation between defect densities as obtained from the charge pumping and current-voltage (I-V) characteristics is investigated before and after hydrogen annealing used to control trap parameters in TFTs. Compared to the charge pumping technique, the I-V method was found to be less sensitive to the variations of Si/SiO2 interface properties.