  For a large class of integrable quantum field theories we show that the
S-matrix determines a space of fields which decomposes into subspaces labeled,
besides the charge and spin indices, by an integer k. For scalar fields k is
non-negative and is naturally identified as an off-critical extension of the
conformal level. To each particle we associate an operator acting in the space
of fields whose eigenvectors are primary (k=0) fields of the massive theory. We
discuss how the existing results for models as different as Z_n, sine-Gordon or
Ising with magnetic field fit into this classification.
