Recently there have been a number of reports indicating concern relating to the effect of porosity, pore size distribution, and pore interconnectivity on the integration of highly porous ultra low-k organosilicate glasses (OSGs) as back-end-of-line (BEOL) interconnect dielectrics. In an effort to address these concerns a number of options to control the skeleton and pore structure of OSGs have been proposed, from adding alternative OSG precursors to alternative porogen precursors. In all these options there is a need to balance pore structure modification with critical film properties such as dielectric constant and mechanical strength. In this context, this paper examines porosity and its impact on film properties for highly porous ultra low dielectric constant films. A series of PDEMS® porous OSG films were deposited by plasma enchanced chemical vapor deposition (PECVD) from DEMS® precursor (diethoxymethylsilane) and porogen ATRP (alpha-terpenine). The percent porosity and pore interconnectivity of these films relative to the dielectric constant were measured by ellipsometric porosimetry (EP) and positron annihilation spectroscopy (PALS) respectively. Porosity and pore-size distribution for films deposited using several different species (structure former or porogen precursors) were examined using EP in an effort to understand the impact of the chemical nature of the precursor on pore morphology. Results from these depositions show that it is possible to deposit films with smaller pores using alternative structure formers (ASFs) with bulky organic groups, although there are tradeoffs with respect to other film characteristics. The addition of a separate porogen (ATRP) to the ASF lowered the dielectric constant and the addition of DEMS® precursor to the ASF/ATRP mix gave the films added structural integrity and mechanical strength. Such a fundamental understanding of structure-property relationships will help support successful integration of these porous OSG films.