We report results of the aggregation behaviour of a highly assymmetric PEP-PEO block copoly-mer in water/DMF mixtures. Both are good solvents for PEO and non-solvents for PEP with a large difference in interfacial tension, y, of PEP with the respective solvent. By using- mix-tures of water and DMF we were able to vary the interfacial tension over a wide range and, accordingly, the aggregation behaviour of the block copolymer. Small angle neutron scattering was used for the structural determination of the micelles. Interfacial tensions were measured by pendant drop tensiometry of a PEP homopolymer in the corresponding solvent mixtures. The aggregation number, P, of the micelles decreases steadily from 120 in pure water to 1 in DMF. A double logarithmic plot of y versus P reveals a linear dependence with a slope of 6/5 for large y. This is in a good agreement with a scaling law predicted for starlike micelles. Some deviations at small P and y are observed most likely due to the limit of the validity range of the scaling approach.