Abstract: Superstring theory on AdS_5 x S^5 is classically equivalent, via the Pohlmeyer reduction, to a fermionic generalization of an extended sine-Gordon model. The Pohlmeyer-reduced and light-cone gauge string S-matrices turn out to be different limits of a larger interpolating S-matrix, based on the q-deformed R-matrix of Beisert and Koroteev, 2008. In this talk I will briefly review the reduction before discussing recent progress in constructing the quantum S-matrix of the reduced and interpolating theories. It is suggested that the interpolating S-matrix may provide a new way to regularize computations in the string theory.