  We construct QED_2 with mass and flavor and an extra Thirring term. The
vacuum expectation values are carefully decomposed into clustering states using
the U(1)-axial symmetry of the considered operators and a limiting procedure.
The properties of the emerging expectation functional are compared to the
proposed theta-vacuum of QCD. The massive theory is bosonized to a generalized
Sine-Gordon model (GSG). The structure of the vacuum of QED_2 manifests itself
in symmetry properties of the GSG. We study the U(1)-problem and derive a
Witten-Veneziano-type formula for the masses of the pseudoscalars determined
from a semiclassical approximation.
