  We show that the recently found S-matrices describing the scattering of
two-particle bound states of the light-cone string sigma model on AdS5 x S5 are
compatible with Yangian symmetry. In case the invariance with respect to the
centrally extended su(2|2) algebra is not sufficient to fully specify the
scattering matrix, the requirement of Yangian symmetry provides an alternative
to the Yang-Baxter equation and leads to a complete, up to an overall phase,
determination of the S-matrix. We then compare the semi-classical limit of the
bound state S-matrices with the universal classical r-matrix by Beisert and
Spill evaluated in the corresponding bound state representations and find
perfect agreement.
