  The leading and the subleading Landau singularities in affine Toda field
theories are examined in some detail. Formulae describing the subleading simple
pole structure of box diagrams are given explicitly. This leads to a new and
nontrivial test of the conjectured exact S-matrices for these theories. We show
that to the one-loop level the conjectured S-matrices of the $A_n$ Toda family
reproduce the correct singularity structure, leading as well as subleading, of
the field theoretical amplitudes. The present test has the merit of being
independent of the details of the renormalisations.
