Poly( 1-dimethylsiloxypentamethyltrisiloxane) (II) has been prepared by anionic ring-opening polymerization (AROP) of 1-dimethylsiloxypentamethylcyclotrisiloxane (I) catalyzed by butyllithium and HMPA. The isomeric polysiloxane, poly(1-hydlido-1-trimethylsiloxytetramethyltrisloxane (IV), has been prepared by AROP of 1-hydrido-1-trimethylsiloxytetramethylcyclotrisiloxane (III) catalyzed by dilithiodiphenylsilanediolate. These are the first regular polysiloxanes that contain both Si-H and T groups. The polymers have been characterized by H-1,C-13,and Si-29 NMR as well as by IR spectroscopy and elemental analysis: Their molecular weight distribution (M-w/M-n) has been determined by GPC versus monodisperse polystyrene standards. M-n has also been determined by H-1 NMR end group analysis. The highly regular microstructures of these linear polysiloxanes has been determined by Si-29 NMR spectroscopy. Every third siloxane unit in these polymers is a RSiO3/2 (T) group. Both copolymers contain reactive Si-H groups that permit chemical modification by hydrosilylation reactions. Quantitative hydrosilylation of II with benzophenone catalyzed by activated dihydridocarbonyltlis(triphenylphosphine)- ruthenium (Ru) yields poly(1-diphenylmethoxydimethylsiloxypentamethyltrisiloxane). Karstedt-catalyzed quantitative hydrosilylation of II with styrene gives copoly(1'-phenylethyldimethylsiloxypentamethyl trisiloxane/2'-phenylethyldimethylsiloxypentamethyltrisiloxane). These modified polysiloxanes also have regular microstructures. On the other hand, chemical modification of TV by hydrosilylation of benzophenone catalyzed by Ru or of styrene catalyzed by Karstedt catalyst proceeds slowly. It was not possible to obtain quantitative chemical modification of IV by either method.