Purpose: The purpose of this paper is to explore the conditions under which a co-design team comprised of in-house professionals and leading-edge service users can generate innovative service design concepts. Design/methodology/approach: The investigation used a field-experimental design to conduct two studies. Observations and open-ended questionnaires were used to examine cross-comparison matrices with experts rating the generated outcomes and t-tests being used to compare the outcome ratings between teams of different compositions. Findings: The outcomes produced by a co-design team seem to be linked to the team diversity – process facilitation relationship. Bringing a variety of knowledge and skills into the team can lead to original outcomes, while a high disparity between members’ backgrounds can require extensive efforts to facilitate a collaborative process. Separation between users’ objectives can result in a user-driven process and outcomes that are too specific for the broader marketplace. Co-design teams that characterize minimum separation, maximum variety, and moderate disparity are likely to produce the most promising results. Research limitations/implications: The research was restricted to a narrowly defined study setting and samples. Future research should replicate the current study in other service contexts using different team compositions. Practical implications: Co-design requires the careful selection of users based on their background and motivations, as well as the facilitation of a process that enables the team to collaboratively transform relevant knowledge into innovative outcomes. Originality/value: The research contributes to a better understanding of the team composition – process facilitation relationship affecting innovation outcomes. Doing so provides a more fine-grained picture of the co-design team composition and the facilitation requirements for service design. © 2018, Emerald Publishing Limited.