BACKGROUND:There is growing evidence that peer support programs may be effective in supporting breastfeeding mothers. A randomised controlled trial (RCT) (the RUBY study) that tested peer support in the Australian context found that infants of first-time mothers who received proactive telephone peer support were more likely to be receiving breastmilk at six months of age. METHODS:This qualitative sub-study of the RUBY RCT explores the experiences and views of peer volunteers who delivered the intervention. Focus groups were conducted with 17 peers from the RUBY RCT between November 2015 and March 2016. All had provided peer support to at least one mother. RESULTS:We found that volunteers identified strongly with the mothers' need for support when establishing breastfeeding. Key components of the support were strengthening the mothers' self-belief through affirmation and sharing experiential knowledge. Volunteers found the role rewarding and personally therapeutic although some women reported challenges initiating and maintaining contact with some mothers. Data were analysed using a hybrid approach to thematic analysis combining inductive and deductive techniques. CONCLUSIONS:Breastfeeding peer support programs are reliant on recruitment of motivated volunteers who can provide empathetic mother-to-mother support. This study provides important information regarding volunteers' experiences that may support the upscaling of breastfeeding peer support for new mothers. TRIAL REGISTRATION:Australian New Zealand Clinical Trials Registry, ACTRN 12612001024831.