The measurement of extensional viscosity, particularly for low viscosity complex fluids, has long been recognised as both an essential and challenging theological task. Many of the techniques currently available have drawbacks that either preclude the use of low viscosity fluids or provide varying levels of applied fluid strain. By combining oscillatory flow with a stagnation point extensional flow field, conditions of steady-state stretching using only tiny volume displacements can be achieved. This extensional flow oscillatory rheometer has four electronically controlled micro-pumps positioned at the end of each channel of a cross-slots flow cell, creating planar extension, a differential pressure transducer records flow resistance measurements. The geometry permits the shear and extensional theological components to be separately determined. An optical probe records simultaneous flow field stability, microstructure and molecular orientation data. Results are presented for dilute (10-100ppm) polystyrene and hyaluronan polymer solutions, showing the capability of the technique for measuring the extensional viscosity and non-ideal Trouton ratios, controlling the fluid strain, following the evolution of molecular strain, assessing molecular flexibility and deriving the molecular weight distribution of high polymers. (c) 2006 Elsevier B.V. All rights reserved.