Micelle formation by several styrene-isoprene two-block copolymers in selective solvents, viz., n-heptane, n-dodecane, N,N′-dimethylformamide, and N,N′ -dimethylacetamide, was studied by viscometry and photon correlation spectroscopy (PCS). Aggregation number and micellar weight were deduced by combining viscosity and PCS data. The temperature effect on the viscosity behavior of micellar solutions of block copolymers in different selective solvents is examined and explained.1H-NMR spectra of the copolymer in n-heptane were recorded at different temperatures to explain the characteristic features of micellar solutions.