We investigated the influence of interfacial tension, gamma, on the micellization properties of a highly asymmetric poly(ethylene-co-propylene)-poly(ethylene oxide) (PEP-PEO) block copolymer in mixed solvents consisting of water and dimethyl form ami de (DMF). Both are good solvents for PEO and nonsolvents for PEP but exhibit, a large difference in gamma with respect to the insoluble core block. Micellar characteristics were obtained by small-angle neutron scattering (SANS) and subsequent fitting of a core-shell form factor to the scattering patterns. The curves are perfectly described by a hyperbolic density profile for the shell, n(r) similar to r(-4/3), indicating a starlike structure of the micelles. The aggregation numbers of the micelles decrease with increasing DMF-water ratio from P = 120 in pure water to nonaggregated chains in pure DMF. Corresponding interfacial tensions were determined by pendant drop tensiometry using a PEP homopolymer of equal molar mass. A correlation of P with gamma reveals a power law dependence, P similar to gamma(6/5) in accordance with the scaling prediction of Halperin for starlike micelles. Additionally, it was found that the addition of DMF leads to a considerable decrease in the micelle radii, which cannot be explained by the decrease in P alone. Measurements of the second virial coefficients, A(2), of a PEO homopolymer by SANS reveal clearly reduced values compared to A(2) in pure water but still good solvent conditions for PEO in all water/DMF mixtures. However, a significant reduction in the radius of gyration was not found. Therefore, it was concluded that the reduced solvent quality has a more pronounced effect for the PEO chain dimensions in the confined geometry of a micellar corona.