While it is known that the
tensor product of two dimension groups is a dimension group, the corresponding
problem for interpolation groups has been open for a while. We solve this problem
here, by proving that the tensor product of two interpolation groups may not be an
interpolation group, even for directed, torsion-free interpolation groups. We also solve
the corresponding problems for refinement monoids (with tensor product of
commutative monoids) and for lattice-ordered groups (with tensor product of
partially ordered abelian groups).