Para-positronium (p-Ps) and ortho-Ps (o-Ps) which are formed in the pores of materials are very sensitive probes to determine pore sizes and type of porosity in porous thin film materials. In addition to Positron Beam Analysis with Doppler Broadening a high resolution technique for measuring momentum distributions of the p-Ps is applied: 2D-ACAR combined with the intense positron beam facility present in Delft. Effusion of hot p-Ps was observed from highly porous low-k dielectric SiO(2) films. In silicon nanocavities p-Ps and even o-Ps appeared to exist, but the fractions were seen to depend strongly on the hydrogen content of the cavities. Hydrogen coverage of the cavity walls leads to effective cooling of the hot Ps. Helium in the nanocavities causes a change in the annihilation of Ps. Models for cooling of Ps and the influence of gas and internal gas coverage are discussed.