We have investigated the thermal degradation of gate oxides in metal-oxide-semiconductor (MOS) structures with Ti-polycide gates. We found that the Ti-diffusion into the underlying polysilicon, and consequently to the gate oxide occurs during thermal cycling processes. In the intermediate stages of the Ti-diffusion, Q(bd), values of the MOS diodes with Ti-polycide gates decrease, which is due to the increase of the trap densities in gate oxides by the diffusion and subsequent incorporation of Ti in gate oxides. As the diffusion of Ti continues, the initial breakdown of gate oxides at the low-fields occur due to the formation of the gross-defects.