Purpose - Research into inter-organisational relationships has been one of the key drivers in the development of services marketing theory. Yet the understanding of the nature of such relationships, and the management of the relationship process, remains limited. Focusing on the development of buyer-seller relationships in an archetypal professional business service, this paper aims to critically examine the nature and format of inter-organisational service relationships. Design/methodology/approach - Research reported in the paper is based on case study research across multiple dyads (n = 7) in the occupational health sector supported by large-scale survey data. Findings - Argues that, rather than adhering to a single format in terms of characteristics or pattern of development, relationships are diverse and complex. A typology of "ideal type" relationship formats, ranging from quasi-transactional to internalised, is proposed. Each of these ideal types is characterised by a unique set of causal and resultant conditions. Research limitations/implications - The paper is based on data from a single, albeit archetypal, professional business service. Consequently future research should address the replicability of the results across other service sectors. Practical implications - The identification of these discrete relationship formats and their key characteristics along a continuum provides an empirical basis on which service professionals can develop targeted strategies for the management of particular inter-organisational relationships. Originality/Value - Building on preceding research, the paper provides empirically based analysis of the nature and format of inter-organisational relationships in professional service markets. © Emerald Group Publishing Limited.