PurposeThe purpose of this paper is to examine the competition between accounting sustainability assurance providers (ASAPs) and non-accounting sustainability assurance providers (NASAPs), and how this competition influences the institutionalization of the evolving field of sustainability assurance.Design/methodology/approachAn interpretivist research methodology, guided by an institutional work perspective, is used to analyze interviews with 15 SAPs and 35 sustainability reporting managers (SRMs) in Australia and New Zealand.FindingsASAPs prefer to use International Standard on Assurance Engagements 3000 (ISAE3000), because it is well recognized in the profession, adheres to ASAPs’ regulatory requirements, and mirrors their financial audit methodologies. This preference influences ASAPs’ institutional work as they compete against NASAPs and how they institutionalize sustainability assurance. ASAPs’ institutional works include presenting sustainability assurance as similar to a financial audit, arguing in support of a single provider for financial audits and sustainability assurance, and undermining NASAPs and their preferred sustainability assurance standard, AA1000 Assurance Standard (AA1000AS), by appealing to senior management. In comparison, NASAPs promote AA1000AS as a specialist standard among SRMs, emphasizing the standard’s sustainability enhancing qualities and its flexibility, while discrediting ASAPs and ISAE3000 as out of touch with sustainability objectives.Research limitations/implicationsA new conceptual model is constructed that can be used in institutional work research.Practical implicationsThe accounting profession is encouraged to consider more flexible, innovative methods in new assurance markets. This involves using new assurance standards as well as developing specialist standards for new forms of assurance. Regulation over sustainability assurance could be helpful, but regulators should be careful not to stifle competition in this evolving field.Originality/valueThis paper examines how competition between ASAPs and NASAPs influences the institutionalization of sustainability assurance. The paper offers a new model for the analysis of institutional work, which could be used by researchers, new insights into the emerging field of sustainability assurance, as well as a figure and discussion that clarifies the broader implications of the findings.