Positron lifetime spectra in ordered mesoporous silica were measured. In the raw material (still containing the template) the voids in micellae reaching 1 nm in size were observed. Ortho-positronium (o-Ps) lifetime was found to be very sensitive to the carbon deposits left in pores after pyrolysis. In highly dispersed materials the relation between the pore diameter and positronium lifetime is shadowed by o-Ps escape from the pores to extragranular spaces. A very slow thermalization of o-Ps and increased lifetime of free positrons in low-density bulk were also observed.