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Entangling properties of a mixed 2-qubit system can be described by the localhomogeneous unitary invariant polynomials in elements of the density matrix.The structure of the corresponding invariant polynomial ring for the specialsubclass of states, the so-called mixed X-states, is established. It is shownthat for the X-states there is an injective ring homomorphism of the quotientring of SU(2)xSU(2) invariant polynomials modulo its syzygy ideal and theSO(2)xSO(2)-invariant ring freely generated by five homogeneous polynomials ofdegrees 1,1,1,2,2.

On the ring of local unitary invariants for mixed X-states of two qubits