Small-angle neutron scattering is used to measure the infinite dilution radius of gyration (R g,0 ) for a range of molecular weights (10-250 kg/mol) of perdeuterated poly(ethylene oxide) (d-PEO) in various imidazolium-based ILs at 80 °C. The Flory exponents (ν) evaluated in the ILs studied (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TFSI]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF 6 ]), and 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM][BF 4 ])) lie between 0.58 and 0.60, indicating good solvent behavior. The coil dimensions of d-PEO increase on increasing the alkyl chain length of the cation and on decreasing the anion basicity. A more pronounced effect on coil dimensions is observed upon altering the anion, compared to the dependence on cation alkyl chain length. The temperature dependence of R g,0 is found to be moderate in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF 4 ]), where the coil size decreases upon increasing the temperature from 80 to 135 °C. Addition of up to 10 wt % of a salt, lithium bis(trifluoromethylsulfonyl)imide, to mixtures of d-PEO and [BMIM][TFSI] has no significant effect on R g,0 . These results are discussed in terms of the relative interactions among PEO, the cation, and the anion.