This paper describes the evaluation of software, software designed to allow visually impaired users to explore the structures of chemical molecules using a speech based presentation. Molecular structures are typically presented as two dimensional schematics, and are an important example of a widely used form of diagram -- the graph. software is designed for exploring this specific class of graph. Among its features is the ability to recognise and make explicit features of the graph that would otherwise need to be inferred. The evaluation compared software with a simpler version without this facility, and found that participants were able to explore molecular structures more easily. We discuss the software, evaluation and results, particularly comparing them with theoretical considerations about how sighted readers use diagrams. Finally, we extract the important issues for non-visual graph presentation: making implicit features explicit; enabling hierarchical and connection-based browsing; allowing annotation; and helping users keep their orientation.
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