The subject of the present paper is an analysis of semantic roles of reason clauses introduced by the central causal conjunctions because, since, as and for. 200 reason clauses have been classified according to their semantic role — either as direct reason clauses or indirect reason clauses. Direct reason clauses have been further subclassified into four subgroups. Consequently, it was found that CGEL does not cover all semantic relationships in practice, since quite a large number of examples do not fit into this classification. Therefore we set up a separate group called ‘Reason and Evaluation/Attitude’. Furthermore, it was demonstrated that the semantic role of a reason clause is closely related to its syntactic function.