Non-linear shear and elongational flow properties of the ternary fluid S1 have been compared to those of a binary solution with similar linear viscoelastic properties. In contrast to the binary solution, fluid S1 exhibits a flow instability. The onset of this instability in rotational flow is characterized by a critical Deborah number De(c) = 13.5. Secondary flow causes an irreversible decrease of zero-shear viscosity. A flow-induced degradation of the dissolved polymer has been proved by different analytical methods. However, this effect is not sufficient to account for the observed reduction of eta0, and it is speculated that the irregular flow additionally gives rise to a phase separation or demixing of the originally homogeneous solution. Up to now there is no direct experimental evidence for this hypothesis. Elongational flow properties have been characterized by means of an opposing jet apparatus. Apparent elongational viscosity of fluid S1 increases strongly with increasing apparent elongation rate epsilon and total strain epsilon. A strain-independent equilibrium value is not reached. In the case of the binary solution eta(E) is independent of e and increases only slightly with increasing epsilon. Strong fluctuations of the force signal are observed in the case of fluid S1 when a critical flow rate is exceeded, indicating secondary flow phenomena.