Abstract Purpose To simulate the Bebig model I125.S16 source and obtain AAPM Task Group Report 43 brachytherapy dosimetry parameters for comparison to consensus and previously published values. The seed model will then be incorporated into a Monte Carlo model of COMS eye plaques and simulation results will be used for seed-carrier set modeling in a commercial planning system. Methods PENELOPE was used to simulate the seed and the applicators for different sizes and loading levels. The corresponding TG-43U1 dosimetric parameters of the seed were calculated. Bebig Plaque Simulator was used. Results The air kerma strength, the dose rate constant and the radial dose and 2D anisotropy functions found showed a good agreement with those published by other authors. Dose distributions were determined for the 12 and 20 mm COMS plaques loaded with a single seed and for the 12 mm plaque fully loaded. The plaque effect on the eye dose and the interseed absorption were evaluated. If the plaque is loaded with a single seed, the dose in the central axis reduces about 10% at 5–6 mm depth with respect to the case in which the plaque is not present. This reduction does not depend on the plaque size. When the plaque is fully loaded, an additional reduction in the dose with respect to the dose in water is observed mainly due to the effect of the Silastic carrier. The mean dose reduction in the central axis of the 12 mm plaque due to the interseed absorption was 0.5%. A new physics file for the planning system was created with the results obtained from the simulations. Results obtained using this adapted model for the 12 mm plaque fully loaded agreed with the corresponding simulation. Dose rate at the prescription point differs 4.7% when the adapted model is used instead of the default model. Conclusions Simulation results for COMS plaques are consistent with those published for other seeds. The planning system studied appears as a good tool for dose calculation in ophthalmic brachytherapy treatments. The new physics model, built up from Monte Carlo results, has been commissioned by comparing calculations made with the planning system to those obtained from Monte Carlo simulations.