The distribution of an ionic liquid within microphase-separated domains of a block copolymer in mixtures of the two components is studied using contrast-matched small-angle neutron scattering (SANS) and differential scanning calorimetry (DSC). In concentrated mixtures of a poly(styrene-block-2-vinyl pyridine) (S2VP) copolymer in an imidazolium bis(trifluoromethane)sulfonimide ([Im][TFSI]) ionic liquid (block copolymer volume fraction ranging from 0.51 to 0.86), the ionic liquid preferentially pervades the poly(2-vinyl pyridine) (P2VP) blocks. Unexpected differences in the degree of partitioning into P2VP-rich and polystyrene-rich (PS) microphases are observed in mixtures with hydrogenated versus deuterated [Im][TFSI]. In the case of mixtures with hydrogenated [Im][TFSI], the microphase partition coefficient, defined as the ratio of the ionic liquid volume fraction in the PS-rich microphase relative to that in the P2VP-rich microphase, ranges from 0.0 to 0.1. In contrast, the microphase partition coefficient in mixtures with deuterated [Im][TFSI] range from 0.0 to 0.7.