The PALS spectra of alkanes and their binary mixtures were measured as a function of temperature and pressure. It was found that in rigid phase positronium locates in the interlamellar gap, while in the rotator phase in the vicinity of nonplanar (kinked) conformers. An interesting feature is equality of o-Ps lifetimes in rotator and liquid phases in spite of different mechanisms of Ps formation. In binary mixture of nonadecane and tricosane an instability of phase transition from rigid to rotator in the range of tricosane admixture (3-10)% was observed. The pressure and temperature dependences of o-Ps lifetime and intensity are similar, 1 K is roughly equivalent to 4 MPa. The effect of “overpressing” the rotator phase was demonstrated in the mixture containing 10% of tricosane. Unstable rotator phase was observed up to pressure of 110 MPa at 300 K, while at this pressure the stable rotator phase needs the temperature over 310 K.