  We construct interacting quantum fields in 1+1 space-time dimensions,
representing charged or neutral scalar bosons at positive temperature and zero
chemical potential. Our work is based on prior work by Klein and Landau and
Hoegh-Krohn. Generalized path space methods are used to add a spatially cut-off
interaction to the free system, which is described in the Araki-Woods
representation. It is shown that the interacting KMS state is normal w.r.t. the
Araki-Woods representation. The observable algebra and the modular conjugation
of the interacting system are shown to be identical to the ones of the free
system and the interacting Liouvillean is described in terms of the free
Liouvillean and the interaction.
