This paper proposes a structured methodology that uses focus group interviews to evaluate Information Visualization techniques and tools. Focus groups are particularly suitable for the collection of qualitative data from users, and allow researchers to uncover unexpected problems that cannot be identified through analytical evaluations or controlled experiments. The approach relies on open-ended questions to explore user attitudes, feelings, and beliefs, but also indicates a number of questions designed to explore specific cognitive tasks related to Information Visualization systems. We argue that focusing the discussion on cognitive tasks allows for the generation of user comments that are more effective than informal and unstructured interviews.