This article investigates assessor decision making when using and applying a marking guide for a note-taking task in a specific purpose English language listening test. In contexts where note-taking items are used, a marking guide is intended to stipulate what kind of response should be accepted as evidence of the ability under test. However, there remains some scope for assessors to apply their own interpretations of the construct in judging responses that fall outside the information provided in a marking guide. From a content analysis of data collected in a stimulated recall group discussion, a taxonomy of the types of decisions made by assessors is derived and the bases on which assessors make such decisions are discussed. The present study is therefore a departure point for further investigations into how assessor decision-making processes while marking open-ended items might be improved.