Pore entrance size of cage-like pores in mesoporous silica films is difficult to be determined by conventional techniques. A simple expedient is proposed by using positron annihilation lifetime spectroscopy (PALS) based on a slow positron beam. Because of the nature of positronium (Ps, the bound state of a positron and an electron) in mesoporous silica, almost no Ps annihilates in the smaller connecting channels of cages. By trimethylsilylation of the silica, an appreciable fraction of Ps can be trapped and annihilate in the channels, which renders the possibility to estimate the pore entrance size from Ps lifetime in it. (C) 2013 Elsevier B. V. All rights reserved.