A model supported analysis of the hydrodynamics and mixing properties of a screw-loop reactor is presented. Two different mathematical models taking into account axial dispersion and recycling are compared, employing boundary conditions according to a Danckwerts or Wehner-Wilhelm. On the basis of velocity measurements and impulse responses it is shown that the boundary condition according to Wehner-Wilhelm is more appropriate for the description of the reactor. Dependence of recycling, residence times and dispersion on operating conditions like feed flow rate, screw rotation number and viscosity are reported and discussed.