We consider solvable Lie groups which are isomorphic to unimodularizations of
products of affine groups. It is shown that a lattice of such a Lie group is
determined, up to commensurability, by a totally real algebraic number field. We
also show that the outer automorphism group of the lattice is represented
faithfully in the automorphism group of the number field. As an application, we
obtain a classification of codimension one, volume preserving, locally free
actions of products of affine groups.