A~traet--Poly(styrene-h-dimethylsiloxane) (PS-PDMS) diblock and poly(styrene-h-isoprene-hdimethylsiloxane) (PS-PI-PDMS) triblock copolymers have been synthesized by living anionic sequential addition polymerization in tetrahydrofuran as solvent with n-BuLl as initiator. These copolymers were characterized by gel permeation chromatography (GPC), membrane osmometry, viscometry, nuclear magnetic resonance (NMR) spectroscopy, small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The universal calibration curve of [q].~, vs GPC peak elution volume in methyl ethyl ketone for polystyrene (PS) and poly(dimethylsiloxane) (PDMS) standards was not unique suggesting that this method cannot be used for the calculation of true molecular weights of these diblock copolymers. Number-average molecular weights (Mo) of the diblock copolymers were computed from their uncorrected GPC chromatograms using log M vs elution volume calibration curves of PS and PDMS in combination with the following equation: log '~bl,,~k = Wps log 'Ql's + WmMs log AtPnMS where Wps and WpnMs are the weight fractions of PS and PDMS in the block copolymer (Wr.s + 14"pi)Ms = 1). For /13.<4 x 104. the calculated values were close to those measured experimentally. Micelles composed of PS-PDMS diblock copolymer formed in heptane were studied using SAXS. Radii of gyration of these micelles varied as the I/3 power of the block copolymer molecular weight. SAXS measurement of the toluene cast films of the PS-PI-PDMS triblock copolymer yielded lamellar thickness of 54 nm. This value was largcr than that (18.5 35 nm) obtained with TEM.