The transfer-admittance of n- and p-channel MOS transistors has been measured under the condition of a uniform channel. These MOS transistors all showed a measurable “slow interface state drift” <0·1–0·2 V. The transfer-susceptance has been found to show a significant peak value in moderate inversion. Over the entire moderate and strong inversion region the transfer-susceptance remains constant as a function of the measurement frequency ω between 1·6 Hz and 2 × 104 Hz, while the transfer-conductance varies almost like ln ω. Furthermore the transfer-susceptance shows a linear relationship with the variation of the transfer-conductance per frequency decade. The paper shows that these phenomena can be well explained by assuming a tunneling of channel charge carriers into electron states in the oxide. Also the temperature behaviour of the transfer-admittance does not seem to be in conflict with this tunnel model. More than the CV measuring method the measurement of the transfer-admittance allows an investigation of the interaction between mobile inversion layer charge carriers, and interface states in the condition of moderate inversion (5 × 1010-5 × 1011 electrons cm−2). The measuring method might therefore find application in the investigation of charge trapping in CCD devices. As a pertinent result the density of oxide states having time constants between 6 × 10−1 sec and 1·6 × 10−5 sec appears to increase to values of about 1011 per cm2V as the interface state energy approaches the conduction and valence band edge energies within a distance of 70 meV.