Assessing the quality of domain conceptual models is a key issue in the early stages of information system development. Several proposals of quality criteria have been proposed in the literature but they are ad-hoc and difficult to compare. In this paper, we propose the use of the ISO/IEC 9126 quality standard as a framework to build a quality model for ER diagrams. We choose from the standard the criteria that are applicable for ER diagrams and then we apply the goal-question-metric approach to obtain metrics for these criteria. The advantages of using this ISO/IEC standard are completeness of the quality model, reuse of knowledge from other disciplines and a better alignment with other quality models that apply to other system development artifacts (as system requirements or the software product itself). As an additional benefit, we can use the obtained quality model to analyze other approaches to ER quality assessment.