The high-low-frequency (HLF) transconductance method is applied to determine the interface state density profile in small-geometry thick- and thin-film (fully depleted) MOSFETs formed on single-and multiple-oxygen implanted (SIMOX) Si substrates. By comparing the real part of the measured high-and low-frequency transconductance and using a static drain current-to-gate voltage measurement on a MOSFET operating in the linear region, the energy distribution of the interface state density is determined without knowing the doping profile in the channel. The results yielded the interface state density in the range of ∼5 to 10 × 10 10 eV −1 cm −2 for single implanted samples and 2 × 10 10 eV −1 cm −2 for multi-implanted samples. Interface state densities determined by the HLF transconductance method were found in good agreement with that determined by the charge pumping (CP) method.