We present a systematic study on the admittance characterization of surface trap states in unpassivated and SiNx-passivated Al0.83In0.17N/AlN/GaN heterostructures. C-V and G/omega-V measurements were carried out in the frequency range of 1 kHz to 1 MHz, and an equivalent circuit model was used to analyze the experimental data. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming models in which traps are located at the metal-AlInN surface. The density (D-t) and time constant (tau(t)) of the surface trap states have been determined as a function of energy separation from the conduction-band edge (E-c E-t). The D-st and tau(st) values of the surface trap states for the unpassivated samples were found to be D-st congruent to (4 - 13) x 10(12) eV(-1) cm(-2) and tau(st) approximate to 3 mu s to 7 mu s, respectively. For the passivated sample, D-st decreased to 1.5 x 10(12) eV(-1) cm(-2) and tau(st) to 1.8 mu s to 2 mu s. The density of surface trap states in Al0.83In0.17N/AlN/GaN heterostructures decreased by approximately one order of magnitude with SiNx passivation, indicating that the SiNx insulator layer between the metal contact and the surface of the Al0.83In0.17N layer can passivate surface states.