Argumentation is central to law. Written and oral argument structures, however, are often difficult to analyze and employ in instruction. Diagrammatic models of argument offer a potential solution to these problems. In this paper we report on the results of an empirical study into the diagnostic utility of argument diagrams in a legal writing context. The focus is on comparing experts' and student-produced argument diagrams and on the extent to which the latter can be used to predict students' performance on subsequent writing tasks. We present the results and draw some tentative conclusions.