The pore connectivity threshold (percolation threshold) in porous low-K dielectric films measured by Positron Annihilation Lifetime Spectroscopy (PALS) reflects only the interconnectivity of mesopores (2–50 nm size). Here we show direct evidence for molecular (toluene) diffusion at porosity values significantly below the PALS's percolation threshold. Therefore, the pores are still interconnected through intrinsic micropores (<2 nm) in the low-K film matrix. This implies that fundamental limitations may exist in the evaluation of pore interconnectivity and the integrity of diffusion barriers deposited on top of porous low-K films by the detection of Ps escape.