The preparation of linear polysilsesquioxanes having Si-OH side groups is not trivial, because direct condensation of multifunctional silanol derivatives would lead to branched or crosslinked polymers. We have successfully prepared an unsymmetrical double-decker silsesquioxane with a silyl hydride function at one end and two silanol groups at the other. Linear POSS polymers having Si-OH side groups are obtained selectively in good yields. The properties and potential applications are briefly presented.