The energy position of an iodine‐related donor has been tracked through the alloyed system ZnSxSe1−x, where 0.60<x<1.00. Both optical and thermal energies have been measured with junction space‐charge techniques. The results suggest that the optical threshold energies and lattice‐relaxation effects are almost independent ofx, although the activation energy of the thermal emission rate increases considerably with increasingx. Detailed measurements of the temperature dependence of the capture cross section for electrons show that the enthalpy of the iodine‐related center is fairly constant for higher values ofx.