The lifetime of ortho-positronium (o-Ps) in solid phases of long-chain n-alkanes was measured. In even-numbered alkanes, only one phase transition is observed; a very long lifetime is related to large free volumes near kink-shaped conformers. In odd-numbered alkanes (n less than or equal to 25), another solid phase (phase IV) is characterized by slightly increased o-Ps lifetime, which means the domination of end-gauche and double-gauche intramolecular defects. In C29H60 the structure of defects changes continuously in the whole range of phase IV. (C) 2004 Elsevier B.V. All rights reserved.