Globally, companies increasingly publish separate general purpose, non-financial (sustainability) reports. Some of these are independently assured and assurers may or may not be from the auditing profession. We seek to understand this emerging voluntary assurance market. Using a sample of 2,113 companies (from 31 countries) that produced sustainability reports between 2002-2004, we use sequential logit analysis to identify the factors associated with the decision to voluntarily purchase assurance and the choice of assurance provider. We hypothesize that a company's need to enhance credibility through assurance and choice of assurance provider will be a function of company-, industry-, and country-related factors. Our results support the argument that companies seeking to enhance the credibility of their reports and build their corporate reputation are more likely to have their sustainability reports assured, although it does not matter whether the assurance provider comes from the auditing profession. We also find that companies operating in stakeholder-orientated countries are more likely to choose the auditing profession as an assurer.