The x‐ray bright BL Lac object PKS2155‐304 was observed with the PSPC on two separate occasions: May 6, 1991 and November 17, 1992. There was a 45% drop in flux between the observations. The later observations were well fit with a simple power law; however, the earlier spectrum appears more complex, and requires an additional feature. We modelled this as an absorption notch or edge. Similar absorption by hot gas, apparently intrinsic to the source, has been detected with other instrumented in this and other BL Lac objects. However, we believe the source of the feature is instrumental and is probably the result of a small temporal gain shift (∼1–2%) in the response matrix. ©1994 American Institute of Physics