We investigate Al2O3- and ZrO2/InAlN/GaN metal-oxide-semiconductor heterostructures (MOS-H) using capacitance-time transients in the temperature range of 25-300 degrees C. A deep-level transient spectroscopy based analysis revealed the maximum interface state density distributions D-it(E) up to 3 x 10(13) and 1 X 10(13) eV(-1) cm(-2) for the Al2O3/InAlN and ZrO2/InAlN interface, respectively. The integral densities of interface states correlate well with the trapping-related gate-lag effect in corresponding InAlN/GaN MOS high electron mobility transistors (HEMTs). This explains the strongly reduced lag effect in ZrO2 MOS HEMTs. We assume hole trapping at oxide/InAlN interface to be a dominant effect responsible for the gate-lag effect in InAlN/GaN MOS HEMTs. (C) 2009 The Japan Society of Applied Physics 10.1143/JJAP.48.090201