BackgroundConsultation with partner expert groups (PEGs) is an important step in updating guidance under European chemical legislation concerning nanomaterials. Here, we briefly review the differences between PEGs and the broader use of expert groups in general, and scrutinise the five closed - and one ongoing-nano-specific PEGs to investigate stakeholder composition, level of engagement and the extent to which stakeholder comments resulted in revisions being implemented in ECHA’s draft guidance.ResultsThirty-six different stakeholders were identified as having been involved in the closed PEG consultations, and an additional nine are currently involved in an ongoing PEG. For the closed PEG consultations, industry and trade associations (I&Ts) and member or associated member states (MSCAs) were the most represented groups, accounting for 15 and 13 members, respectively, whereas non-governmental organisations (NGOs) and European Union bodies (EUB) accounted for four members each. Interestingly, Academia was not represented. A total of 2700 comments were provided to ECHA’s draft guidance updates. Of these, MSCAs, I&Ts, EUB and NGOs accounted for 924, 876, 771 and 126 comments, of which 678, 494, 547 and 70 were adopted by ECHA, respectively. Eight stakeholders did not provide a comment.ConclusionsEven though EGs and PEGs are not fully comparable, we find that they hold many similarities. The nano-specific PEGs are influenced by a few very active stakeholders that have the time, resources and motivation to engage extensively while some stakeholder groups are partly or completely missing. We recommend that ECHA provides funding opportunities for less resourceful stakeholders, in order to minimise the effects of scarce funding on engagement. Furthermore, we recommend broadening the list of accredited stakeholder organisations, thereby allowing for more diversity among stakeholders involved, e.g. Academia, and that ECHA provides a justification for inclusion of the PEG members.