Microstructural studies of silica gel powder were carried out using positron annihilation lifetime spectroscopy (PALS) in conjunction with transmission electron microscopy (TEM). It is argued that the two distinct long-lived components found (labeled by tau(3) and tau(4)) may be ascribed to ortho positronium annihilation in microcavities within the grains and intergranular mesoscopic pores, respectively. In the latter type of void, a significant fraction decays via the three-photon mode. A simple physical picture of positronium annihilation in the larger pores is put forward, while the situation vis-a-vis the smaller cavities is shown to be well described by a modification of the currently prevailing model for the pick-off process. The simple parametrization finally arrived at provides a sharpening of the use of the positron as a useful probe for microstructural study of porous substances. It is emphasized that two different positron annihilation mechanisms prevail in the microcavities and mesopores.