  We consider the two-dimensional dilute q-state Potts model on its first order
phase transition surface for 0<q\leq 4. After determining the exact scattering
theory which describes the scaling limit, we compute the two-kink form factors
of the dilution, thermal and spin operators. They provide an approximation for
the correlation functions whose accuracy is illustrated by evaluating the
central charge and the scaling dimensions along the tricritical line.
