The infinite dilution radii of gyration (R-g,R-0) for five different molecular weights (10-250 kg/mol) of perdeuterated poly(ethylene oxide) (d-PEO) have been determined at 80 degrees C in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) using small-angle neutron scattering (SANS). The results establish the dependence of on polymer molecular weight as R-g,R-0 similar to K-w(0.55). An excluded volume exponent of v approximate to 0.55 indicates that [BMIM] [BF4] is a moderately good solvent and that PEO remains a flexible random coil in this ionic liquid. These results clarify the uncertainty surrounding PEO coil dimensions in this ionic liquid, as computer simulations at various levels of complexity have led to conflicting results, and prior experimental results also do not present a completely consistent picture.