Positron lifetime spectra in solid long-chain alkanes were measured as a function of the storage time with positron source between the samples, and as a function of temperature and pressure, Large differences of the time constant of I(3) increase were observed for odd-numbered and even-numbered molecules. High pressures also induced the I(3) instability, but in this case the I(3) intensity decreased. In the rigid solid phase the o-Ps lifetime in even-numbered alkanes was shorter than in odd ones, which was due to different thickness of the empty layer between the lamellas in the crystal structure.