  The location of the spectrum and the Riesz basis property of well-posed
homogeneous infinite-dimensional linear port-Hamiltonian systems on a 1D
spatial domain are studied. It is shown that the Riesz basis property is
equivalent to the fact that system operator generates a strongly continuous
group. Moreover, in this situation the spectrum consists of eigenvalues only,
located in a strip parallel to the imaginary axis and they can decomposed into
finitely many sets having each a uniform gap.
