  We derive the S-matrix for the d(2,1;alpha)^2 symmetric spin-chain of
AdS3/CFT2 by considering the centrally extended su(1|1)^2 algebra acting on the
spin-chain excitations. The S-matrix is determined uniquely up to four scalar
factors, which are further constrained by a set of crossing relations. The
resulting scattering includes non-trivial processes between magnons of
different masses that were previously overlooked.
