  The exact eigenvalues of the infinite set of conserved charges on the
multi-particle states in affine Toda theories are determined. This is done by
constructing a free field realization of the Zamolodchikov-Faddeev algebra in
which the conserved charges are realized as derivative operators. The resulting
eigenvalues are renormalization group (RG) invariant, have the correct
classical limit and pass checks in first order perturbation theory. For $n=1$
one recovers the (RG invariant form of the) quantum masses of Destri and
DeVega.
