A deterministic diffusion-kinetic model has been successfully applied to the radiation chemistry occurring in a typical spur produced in the gamma-radiolysis of liquid cyclopentane, cyclohexane, and cyclooctane. The predictions of the yields of the cycloalkenes, bicycloalkyls, and the cycloalkyl iodides in solutions of iodine are in excellent agreement with experimental data. The major adjustable parameters in the model are the characteristic radii of the initial Gaussian spatial distributions of the reactive species. Values for these radii were found to be 0.5, 1.1, and 0.55 nm in cyclopentane, cyclohexane, and cyclooctane, respectively. The results suggest that the spurs of cyclopentane and cyclooctane are very small, ca. one molecular diameter, with resulting large local concentrations of reactants. With cyclohexane, the spur size is twice as large and the initial local concentrations are an order of magnitude smaller. The experimentally observed temporal invariance of the cyclohexyl radical can be explained by competing effects in the spur evolution. Details and implications of the spur model are discussed.