A newly modified correlation equation between defect-related positron lifetime tau(2) (ns) determined within two-state trapping model and corresponding radius of free-volume defects R (angstrom) in a full non-linear form tau(2) = 0.285 . [1-R/R + 5.50+1/2 pi.sin(2 pi R/R + 5.50)](-1) or in a simplified linear form tau(2) = 0.212 . (1 + 0.244.R) is proposed to account for compositional trends in free volume evolution of chalcogenide compounds like binary As-S/Se glasses. Specific chemical environment of voids possessing an effective negative charge owing to difference in the electronegativities of atoms forming covalent chemical bonds belonging to neighboring network polyhedrons is shown to play a decisive role in this correlation, leading to larger open volumes than would be attributed to the same positron lifetimes in a typical polymer material. (C) 2014 Elsevier B.V. All rights reserved.