A mechanism limiting the lifetime of high-power millimeter wave windows is the degradation of the ceramic which is exposed to gas. We first observed this phenomena on the air side (atmospheric pressure) of a plane circular waveguide window (Al2O3 ceramic, ∅ = 70 mm) of a TE0,3 mode gyrotron, which was operated with a series of 100 ms pulses at a power of about 100 kW . This degradation occured at the center of the window in an almost circular symmetry. Figure 1 shows the damage pattern at the time of first observation. It consisted of several thin concentric circles (width ≈ 0.3 mm, diameter ≈ 0.5, 2.5, 4.6, 6.7, 8.8 mm) which where not completely closed. From their radial location and from the window thickness one may conclude a TE0ln (or TM0,n) mode with n ≈ 19 being involved in the damage process. It is also observed that the circles occur only near the inner maximum of the TE0,3 mode power density. With further operation of the gyrotron the damage grew, the fine structure smeared out, and the resulting pattern approximately corresponded to the inner maximum of the TE0,3 mode power density (Figure 2).