This paper investigates the variation between if and whether (e.g. I don’t know if/whether you are right), on the grounds that prior studies on complementation in English have not sufficiently addressed the variation between these two complementizers. Based on data extracted from the British component of the International Corpus of English, the results from the study show that although if and whether are mostly interchangeable complementizers, there are grammatical, semantic and stylistic constraints governing the choice of conjunction. The results suggest that a number of factors can influence the choice of complementizer, such as the matrix verb or the presence of an explicit alternative in clauses introduced by the conjunction or. In addition, extralinguistic factors such as age and sex are also revealed as playing a role in the choice of complementizer.