  We describe few aspects of the quantum symmetries of some massless
two-dimensional field theories. We discuss their relations with recent
proposals for the factorized scattering theories of the massless $PCM_1$ and
$O(3)_{\theta=\pi}$ sigma models. We use these symmetries to propose massless
factorized S-matrices for the $su(2)$ sigma models with topological terms at
any level, alias the $PCM_k$ models, and for the $su(2)$-coset massless flows.
