Abstract The internal bremsstrahlung (IB) accompanying the second-forbidden nonunique electroncapture decay of 59Ni has been studied in detail. The IB spectrum measured in coincidence with cobalt K X-rays has yielded the probability ratio of (8.0±0.1) × 10 −4 for the 1s component of IB to the nonradiative K-capture for the energy range from 195 to 1065 keV. Singles measurements have been performed to study the shape of the total IB. The results have been compared with three different models. It was found that the most advanced model, the theory of Zon and Rapoport, underestimates the intensity of the radiation by a factor of 2. The value of the β + branching has been estimated to be (4.2 ± 1.3) × 10 −7.