The reinterpretation of the traditional reference service in an online context is the virtual reference desk. Placing reference services into an online setting, however, presents many challenges. We report a study and analytic framework which addresses support for decision-making during virtual enquiry work. Focusing on specialist law-libraries, the study shows that enquirers do not volunteer important information to the service and that asynchronous communication media and some social obstacles present barriers to prompting. Also, previous enquiries are frequently used to inform current enquiry strategies but barriers exist in accessing this information. We conclude that email is an inadequate medium for supporting virtual reference services, and that system should support automatic, speculative matching between new enquiry content and integrated enquiry knowledge bases. The contribution of the framework is to offer a structured approach to evaluation in multiple virtual reference contexts and enable rapid convergence on barriers to efficient and effective service.