ortho-Positronium lifetime and intensity were measured in a set of binary mixtures of C19H40 and C21H44. In the lamellar structure of solid alkanes, in the low-temperature 'rigid' phase positronium locates between the lamellae. For mixing ratios C-21/C-19 < 0.2 the ortho-positronium lifetime in rigid crystal rises with C21H44 content as a result of increasing the average interlamellar gap width. (C) 2009 Elsevier B.V. All rights reserved.