This paper explores the expression of certainty in a set of high- and low-rated Geography argumentative essays. Using one aspect of Hunston's (1989, 2000) concept of the evaluation of status, the paper compares the statement types used to construe the writer's expression of certainty in the construction of arguments. Drawing on the framework, linguistic expressions that use different statement types are presented in the light of their rhetorical functions to illustrate their role in the development of arguments. Generally, high-rated script writers make more frequent use of particular statement types, including a type of factual statement, "interpretation-support" and "interpretation-mean" statements. The latter two categories are used to reinforce the importance of evidence provided in an argument. Implications for the teaching of certain statement types and their linguistic expressions are also discussed.