The time‐independent scattering theory associated with the non‐self‐adjoint matrix Hamiltonians H of arrangement channel quantum mechanics is presented in detail first using the 3‐particle case as an example. A key feature is the biorthogonality of a suitably constructed set of scattering eigenvectors and duals. Channel space Möller operators, S‐ and T‐matrices are defined and a variety of properties investigated including the way multichannel unitarity is embedded into the theory. Some remarks on the time‐dependent theory are also made. A detailed discussion of channel space density matrix scattering theory(of interest, e.g., in reactive kinetic theory) is presented using the Liouville representation. We describe some special cases includi...