Abstract In a Knudsen flow of polyatomic molecules, non-spherical molecule-surface interaction can give rise to polarization of the angular momenta. This phenomenon is studied for various gases flowing through a Au-coated channel, by observing the magnetic field effect on the flow. Different types of polarization are found to be produced in different types of molecules. This is qualitatively explained with simple models for a molecule-wall collision. The effects in strongly polar gases are found to be very small. This is attributed to non-instantaneous scattering.