A combined 3D numerical/experimental method for the evaluation of viscoelastic constitutive equations is described. The method is applied to a three-dimensional (3D) stagnation flow, which exhibits strong elongational deformations. A polyisobutylene solution is investigated. Laser Doppler anemometry and flow induced birefringence are used to measure pointwise velocities and linewise tin the depth of the flow cell) integrated stresses, respectively. The numerical simulations are decoupled: the 3D velocity field is derived from finite element calculations with the viscous Carreau model, whereafter the viscoelastic stresses are calculated with a four mode Giesekus and Phan-Thien-Tanner (PTT) model. The validity of this approach is checked by comparing the calculated and measured velocities. The optical FIB signal is calculated from the integrated stresses, using the small retardance approximation. The approach is first evaluated for two slit flows with different depth/height ratios (8:1 and 2:1). Calculated velocities and stresses show a good agreement with measurements. In the stagnation flow device, the calculated velocity field diverges from measured velocities at the higher Weissenberg-numbers, at all Weissenberg numbers both viscoelastic models predict much too low stresses near the stagnation line. A modification of the PTT model is proposed to improve the stress predictions in the elongational flow domain.