The optical and mechanical responses of dilute and semidulute solutions of flexible polymer molecules to extensional flow are measured. The optical property of interest, the birefringence, is sensitive to the local orientation of the polymer coil, whereas the mechanical property investigated, the effective extensional viscosity, is sensitive to the overall deformed length of the molecule. These two properties are simultaneously measured as a function of the rate of strain by using an opposing jets apparatus. Solutions ranging from 50 to 300 ppm by weight polystyrene dissolved in both tri-cresyl phosphate, a good solvent at 22°C, and di-octyl phthalate, a theta solvent at 22°C, were studied. The results show that the response of the flexible polymer is dependent on the solvent quality. Both the local orientation and the deformed length of the molecule increase with increasing strain rate at low rates of strain for both solvents. However, in the theta solvent at high rates of strain the birefringence saturates, while the effective extensional viscosity drops with increasing strain rate. This indicates a decrease in the overall deformed length of the molecule at high strain rates due to the decrease in the residence time of the coil in the flow field. In the good solvent, molecular entanglements begin to affect the extensional viscosity at high strain rates if the concentration and molecular weight are sufficiently large. The response of flexible polymers to extensional flow is qualitatively compared to numerical simulations based on bead-spring and bead-rod models. Although these models are able to capture the saturation of birefringence under conditions where the extensional viscosity is still changing, they do not predict the observed maxima in extensional viscosity as a function of strain rate.