The bulk morphological behavior of a sequence of poly(styrene-b-isoprene-b- styrene-b-ethylene oxide) (SISO) terpolymers with fS ≅ f I and fO between 0 and 30%, was reported. Nine SISO tetrablocks were generated by the reinitiation and polymerization of ethylene oxide froma single batch of hydroxylterminated SIS copolymer. Molecular weights and polydispersities were determined by size exclusion chromatography. The block volume fractions were obtained by 1H NMR and are referenced to published homopolymer densities at 140°C. 0.5 wt % of BHT antioxidant was added to the block polymers prior to structural characterization. Morphological assignments were made by utilizing a combination of small-angle X-ray scattering (SAXS), dynamic mechanical spectroscopy (DMS), and transmission electron microscopy (TEM). The SAXS patterns produced by SISO tetrablocks are strikingly different than the other samples, each containing at least three well-defined scattering peaks.