dielectric relaxation studies of a number of diaryl molecules dispersed in atactic polystyrene and in some cases as solid discs have been carried out. preparation of the polystyrene matrices and the solid disks, and the dielectric measurements using a general radio 1615“a capacitance bridge and a hewlett-packard q-meter with appropriate temperature-controllable cells have been described. the experimental data as a function of frequency at different temperatures were subjected to analysis by a series of computer programmes written in the apl language. the activation energy barriers for the dielectric relaxation processes were obtained by the application of the eyring rate equation. a number of diary! molecules with different central bridging atom or group, and different ring substituents have been studied. attempt have been made to identify and study a unique type of rapid, intramolecular rotation process termed "double-internal rotation", in which the two aryl rings undergo simultaneous, coupled rotation. studies have been made to correlate molecular properties such as symmetry, and electronic and sterlc effects to the occurrence of double-internal rotation. symmetric diaryl molecules such as sulfones, sulfoxides, and dibenzothiophenes were found to undergo predominantly over-all molecular rotation in polystyrene matrices. asymmetric diaryl ethers and ketones displayed a major contribution to dielectric relaxation from ovei*-al 1 molecular rotation. symmetric diaryl ethers, ketones, and sulfides (e.g., diphenyl ether, benzophenone, and diphenyl sulfide) were found to undergo primarily double-internal rotation, both in polystyrene and as compressed solids. some large molecules such as 1,2-diphenylcyclopropenone underwent co-operative motion with the polymer chains near the glasstransition temperature of polystyrene. a linear correlation was observed between the enthalpy of activation and the entropy of activation for molecules undergoing the same type of relaxation process. there was also found to be a linear correlation between the volume swept out during molecular reorientation and the energy of activation for the process in rigid symmetric diaryl molecules. energy barriers for the molecular rotation of non-planar symmetric diaryl molecules (e.g., diphenyl sulfone and diphenyl sulfoxide) were found to be >= 60 kj mol [superscript -1] depending on the size of the substituents. planar molecules the size of-dlbenzothlophene were found to have energy barriers for molecular rotation of about 40 kj mol [superscript -1]. the energy barrier for symmetric diaryl molecules which undergo double-internal rotation were observed in most cases to be in the range 5 15 kj mol [superscript -1] at temperatures below 100 k.