User is trying to assign spatialrefence to the input geometries before he intersects them. If don't assign any spatialreference information then intersect will run successfully. At ArcGIS 9.1, calling the following two lines is causing ITopologicalOperator::Intersect to fail: origGeometry.SpatialReference = mSpRef; -> found in GeometryRepair function. origGeometry.SnapToSpatialReference();If you comment out those two lines then the Intersect method will run successfully and will create a valid output.