Certain combinations of commercially available ConFlat type flanges with a gasket can yield an excessively wide gap between the gasket's outer rim and the flange's gasket-retaining counter. (The present ISO Technical Specification for the ConFlat type sealing system allows a maximum gap of some 0.8 mm width.) Tightening this type of flange-and-gasket combination almost never results in a perfect coaxial arrangement; rather, it produces eccentric arrangements of the matching flanges and sandwiched gaskets. In a tightening test with coaxially arranged flanges and eccentrically arranged gaskets, seal performance was independent of the gap width, whether or not baking was included in the test conditions. The results indicate that the existing "capture model" lacks validity as a rationale for the seal mechanism of the ConFlat system. By contrast, the eccentric arrangement of matching flanges did prove to have a strong influence on seal reliability, particularly where the tightening test includes a baking process. The negative impact on seal reliability observed in eccentric arrangements can be ascribed to local bending that occurs on the tightened gasket and the consequent diminution of seal area and seal pressure. Annealing of the gasket material during the baking process lowers seal pressure further, thus increasing the probability of leaks. (C) 2008 American Vacuum Society.