The molar mass and molar mass distribution of polymers subjected to chain scission in planar elongational flow are profoundly affected by the inertial character of the flow, as quantified by the Reynolds number. The degradation of dilute poly(ethylene oxide) (PEO) chains in aqueous-based solvents of varying viscosity was quantified in the planar elongational flow of a cross-slot flow device by gel permeation chromatography with multiangle laser light scattering detection. At low Reynolds number (Re < ∼1000), the steady-state weight-average molar mass, Mw,f, of the scission product distribution scaled with the nominal applied strain rate, ε̇, as ε̇ ∞ Mw,f-1.93±0.15 for PEO/viscous solvent system and ε̇ ∞ Mw,f-2.25±0.10 for the PEO/water system. At greater Reynolds number (Re > ∼1000), the observed scaling was ε̇ ∞ Mw,f-1.04±0.07. Differences of this kind, first quantified by comparing results from stagnation point elongation flows and contraction flows, have previously been attributed to different molecular mechanisms of scission. Yet, our observations in different Reynolds number regimes suggest an alternative explanation based on fluid mechanics for the difference in steady-state scaling exponent. Measurements of pressure drop across the cross-slot flow support this alternative hypothesis.