Integrated circuits that have improved functionality and speed in a smaller package and that consume less power are desired by the microelectronics industry as well as by end users, to increase device performance and reduce costs(1-4). The fabrication of high-performance integrated circuits requires the availability of materials with low or ultralow dielectric constant (low-k: kless than or equal to2.5; ultralow-k:k=2.0) because such dielectrics not only lower line-to-line noise in interconnect conductors, but also minimize power dissipation by reducing the capacitance between the interconnects(1-4). Here we describe the preparation of low- and ultralow-k nanoporous organosilicate dielectrics from blends of polymethylsilsesquioxane (PMSSQ) precursor with globular ethyl acrylate-terminated polypropylenimine dendrimers, which act as porogens. These dendrimers are found to mix well with the PMSSQ precursor and aft er their sacrificial thermal decompositions result in closed, spherical pores of <2.0 nm radius with a very narrow distribution even at high loading. This pore size and distribution are the smallest and the narrowest respectively ever achieved in porous spin-on dielectrics. The method therefore successfully delivers low- and ultralow-k PMSSQ dielectric films that should prove very useful in advanced integrated circuits.