Up to now, the role of divalent ytterbium ion has been controversially discussed in the literature concerning its influence on the photodarkening of ytterbium doped high power laser fibers. In general, however, the experimental findings are relatively sparse and some discussions are based more on speculations than on examined facts. Here we report on systematical investigations concerning the formation of Yb2+ during the fabrication process of preforms and fibers. By Modified Chemical Vapor Deposition, fibers with different codopants (additional to the active ytterbium doping) have been prepared in a well-defined manner, regarding process parameters and glass composition. The comprehensive characterization of the samples involves the ytterbium absorption in the NIR, the UV absorption and UV excited emission. The typical spectral features in the UV and visible range have been analysed and correlated with the presence of Yb2+. The amount of formed divalent ytterbium ions shows a strong dependence on the process route and varies remarkably with the kind and concentration of the codopants. Photodarkening tests have been accomplished in order to correlate the power stability with the Yb2+ content. Moreover, the formation of Yb2+ during the process of UV radiation darkening was investigated.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.