Concept mapping is a powerful learning technique that can be enhanced by computer technology. Software tools are already available for the preparation of concept maps but as yet, few systems provide feedback on their content. The claim made by this article is that by enlisting the student as an assessment partner, computer-based feedback becomes both feasible and useful. The article reports on the development of a semi-automated software analyser for formatively assessing students' concept maps. The analyser is fallible: many of its assessments are initially incorrect. Crucially however, it is also reasonable: a student is able to argue for a more favourable assessment. The article reports classroom trials which indicate that high-school students' experience of concept mapping is enhanced by use of the analyser. The system's wider significance is that it points towards a new genre of educational software based upon a more genuinely dialogic model of human-computer interaction.