The non-linear viscoelastic behaviour of a polyurethane (PUR) network is determined by continuous uniaxial tension tests and stepwise relaxation tests. Following the concept of internal variables on the modelling side, the finite Neo-Hookean material model combined with linear evolution equations for the internal variables is applied to include the time-dependence caused by viscoelasticity. The parameters are identified by an evolution strategy combined with a non-linear finite element analysis which solves the boundary value problem given by the specimen geometry and testing conditions. In conclusion, the combination of the described experiments and modelling provides a full description of the mechanical behaviour of the given PUR.