The effective viscosity and shear behavior of ultrathin films of two structurally different perfluoropolyether fluids have been investigated. The materials used were Fomblin Z, a linear random copolymer of fluoroethylene oxide and fluoromethylene oxide, and Fomblin Y, a branched random copolymer with fluoropropylene oxide and fluoromethylene oxide monomer units. The shearing experiments were conducted with the fluids confined between molecularly smooth surfaces at shear rates ranging from 200 to 4 × 103 s -1. It was found that when the thickness of the fluid films decreases from 10 to 2 nm, both perfluoropolyethers exhibited a sharp increase in viscosity, from bulk values to "surface viscosity" values that are many orders of magnitude larger. With increasing shear rates, the Z-type fluid showed a gradual decrease in the shear stress indicating an apparent ordering of the molecules due to the applied shear forces. On the other hand, with the Y-type copolymer, the shear stresses were significantly lower and were proportional to the shear rate resembling Newtonian fluids. The results are explained in terms of the differences in the molecular architecture of the fluids and suggest a close relation between the molecular structure of the polymer melt near a solid wall and its frictional and lubricating properties. © 1991 American Institute of Physics.