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The magnetic properties of cobalt metal nanowires grown by electrodeposition in porous membranes depend largely on the synthesis conditions. Here, we focus on the role of electrolyte additives on the magnetic anisotropy of the electrodeposited nanowires. Through magnetometry and internal field nuclear magnetic resonance (IF NMR) studies, we compared both the magnetic and crystalline structures of 50 and 200 nm diameter Co nanowires synthesized in the presence or absence of organic additives. The spectral characteristics of IF NMR were compared structurally to X-ray diffraction patterns, and the anisotropy of the NMR enhancement factor in ferromagnetic multidomain structures to magnetometry results. While the magnetic behavior of the 50 nm nanowires was dominated, as expected, by shape anisotropy with magnetic domains oriented on axis, the analysis of the 200 nm proved to be more complex. 59Co IF NMR revealed that the determining difference between the samples electrodeposited in the presence or in absence of organic additives was not the dominant crystalline system (fcc or hcp) but the coherent domain sizes and boundaries. In the presence of organic additives, the cobalt crystal domains are smaller and with defective grain boundaries, as revealed by resonances below 210 MHz. This prevented the development in the Co hcp part of the sample of the strong magnetocrystalline anisotropy that was observed in the absence of organic additives. In the presence of organic additives, even in nanowires as wide as 200 nm, the magnetic behavior remained determined by the shape anisotropy with a positive effective magnetic anisotropy and strong anisotropy of the NMR enhancement factor., NMR equipment at ESPCI Paris is funded in part by the Paris Region. This project has received funding from the European Union′s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 754387. Work at Georgetown University has been supported by the US NSF (ECCS-1933527). The acquisition of a Magnetic Property Measurements System (MPMS3) at GU, which was used in this investigation, was supported by the US-NSF (DMR-1828420). Bruno Bresson (ESPCI) is thanked for performing the SEM analysis., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).