Small-angle X-ray scattering experiments were used to characterize the microphase separation transition (MST) of a poly(styrene-ö-ethylenebutene-ö-styrene) triblock copolymer with a total molecular weight 21 400 and containing 28.5 wt % of styrene. The scattering profiles observed in the homogeneous phase are compared to the predictions of Olvera de la Cruz and Sanchez for an (AB)” star block copolymer with η = 2 arms. The experimental data are found to differ from these predictions in two respects: (1) The dominant wavelength of the concentration fluctuations is found to be temperature dependent in the disordered state; (2) the magnitude of this length exceeds the theoretical prediction. The experimental scattering profiles can be reproduced acceptably by empirically adjusting the value of iV0, the number of monomers in each arm, assumed in the calculation. The resultant interaction parameters obtained by this procedure lead to a spinodal value of ca. 14.9, in reasonable agreement with the prediction of ca. 15.0. Pressure-volume-temperature measurements indicate that the MST does not involve a significant change in volume. © 1990, American Chemical Society. All rights reserved.