Extensive characterizations, mainly by wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) techniques, are reported for co-crystalline (CC) poly(2,6-dimethyl-1,4-phenylene)-oxide (PPO) films with many different low-molecular-mass guest molecules. These characterizations are also reported for the corresponding nanoporous crystalline (NC) phases, as obtained by suitable guest-removal procedures. Two well-separated NC forms, hereafter named alpha and beta, are obtained by guest removal from two well-separated groups of CC forms. alpha and beta NC forms can be easily recognized by reflections in WAXD patterns as well as by suitable FTIR crystalline peaks. Density and degree of crystallinity measurements confirm that both NC phases exhibit a density definitely smaller than for the amorphous phase (rho(am) = 1.04 g/cm(3) rho(beta) = 0.95 g/cm(3) > rho(alpha) = 0.93 g/cm(3)). Density functional theory calculations combined with geometrical analysis on PPO model systems indicate conformations suitable to fit the observed chain periodicities of the two crystalline forms (c = 5.28 and 5.47 angstrom, for alpha and beta NC forms, respectively).