Planar contractions within a microfluidic environment have shown some potential to be used as a simple, cheap extensional rheometer. In this article, flow within a planar contraction is analyzed using an energy minimization calculation with a measure of slip included. The effect of slip on the calculated flow-field is used to show how the apparent extensional viscosity compares with the true extensional viscosity. In particular, it was found that the apparent planar extensional viscosity would be difficult to determine reliably at either low or high Weissenberg numbers but could be distinguished to some extent at moderate Weissenberg numbers between 0.25 and 1. It was also shown, however, that an unknown extent of slip at the vortex boundary would cause an uncertainty of a factor of 2 in any apparent measure of rheological properties. (C) 2015 The Society of Rheology.