Steady shear theology of entangled polystyrene (PS)-diethyl phthalate (DEP) solutions are investigated using mechanical rheometry and optical birefringence measurements. Polymer solutions are formulated using narrow molecular weight distribution polymers that cover a broad range of molecular weights (3.8 x 10(6) less than or equal to M-w less than or equal to 20.06 x 10(6) g/mol) and concentrations of 0.025 less than or equal to phi less than or equal to 0.26. In polymer systems with N/N-e > 12, cone-and-plate and narrow-gap parallel plate shear flow measurements reveal an unusual transition to non-Newtonian behavior at Wi much less than 1. The transition is evidenced by early power law eta similar to (gamma) over dot(-beta) (beta = 0.45 +/- 0.15) deviations from Newtonian fluid response, and is consistent with earlier observations reported by Bercea et al. [Macromolecules 26, 7095-7096 (1993)] and by Islam and Archer [J. Polym. Sci., Part B: Polym. Phys. 39, 2275-2289 (2001)] for solutions of ultrahigh molecular weight polymers in good solvents. The power-law flow regime extends well beyond Wi = 1 for most systems studied and its breadth is proportional to the number of entanglements in solution. Arguments ranging from supramolecular complex formation in concentrated polymer solutions to tube distortion at low shear rates have previously been advanced to explain this behavior. Using gap-dependent steady shear measurements and a slip constitutive equation suggested by theory, we show that the behavior is caused by interfacial slip. A procedure based on attachment of micron-sized silica glass beads to parallel-plate, cone-and-plate, and Couette shear fixtures is shown to be highly effective for reducing slip errors in steady shear rheometry of entangled PS/DEP solutions. Steady-state shear stress and first normal stress difference results obtained using several well entangled PS/DEP solutions are compared with predictions from two recently proposed molecular-based constitutive models.