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NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Single crystals of ester-urea, ketone-urea, and ether-urea inclusion compounds were X-irradiated and investigated by electron spin resonance. Eleven inclusion compounds formed between urea and the straight-chain alkyl esters yielded radicals of the type [...]. The [alpha] and [beta] proton coupling constants of two radicals were studied over the temperature range 352?K to 7?K. The splittings caused by the alcohol (R') protons were resolved in many cases. Information regarding the structure of the inclusion compounds was obtained. The long-lived radical [...] was detected in all six ketone-urea crystals. The spin density in the 2p orbital adjacent to the carbonyl group is 0.81 [plus or minus].04. The ether radicals were of the type [...] and the spin density on the carbon atom is approximately 0.70 [plus or minus].08. The ketone radical and ether radical spin distributions obtained from the [pi]-electron theory are in qualitative agreement with the experimental spin distributions. In addition, single crystals of fumaric acid-urea were X-irradiated and studied by electron spin resonance. The dominant radical [...] in this case is formed by addition of a hydrogen atom (rather than by removal of a hydrogen atom, as above). The fumaric acid-urea crystal is not a hexagonal inclusion compound. The major orientations of the radical were determined with respect to the external crystal morphology. Electron spin resonance studies of the triplet state of pyrene and triplet exciton states in ion radical salts are also briefly reported. The approximate g values and zero field parameters for pyrene in a fluorene matrix at 100[degrees]K are g[subscript xx] = 2.0033, g[subscript yy] = 2.0026, g[subscript zz] = 2.0033, D(xy)/hc = [plus or minus] 0.0806 cm[superscript -1], D(z)/hc = [plus or minus] 0.0810 cm[superscript -1], and E/hc = [plus or minus] 0.0182 cm[superscript -1]. The estimated errors in the g values, D, and E are [plus or minus] 0.005, [plus or minus] 0.0012, and [plus or minus] 0.0009, respectively. The relation between triplet excitons and magnetically dilute radicals in the [...] and [...] ion radical salts was investigated. The temperature dependent broadening of the radical line is apparently caused by an exciton-radical exchange interaction. At 77?K the exciton line width is proportional to the free radical impurity line width, which suggests that unresolved nuclear hyperfine interactions contribute greatly to the exciton line width at this temperature.