Software quality is a many-faceted concept that depends on the kind of artifact to be measured, the context where measurement takes place, the quality framework used, and others. Furthermore, there is a great deal of standards, white papers, and in general proposals of any kind related to software quality. Consequently, a unified software quality framework seems to be needed to compare, combine or select these proposals and to define new ones. In this paper we propose a MOF-compliant approach for structuring quality models in order to formalise software quality issues and deal with quality information modelling. We propose two types of models: a generic model, situated in the M2 MOF layer; and a hierarchy of reference models, defined in the M1 and M0 MOF layers. The generic model elements are derived from the UML metamodel by specialization. Then, we can instantiate them to get reference models that formalise (combinations of) existing proposals which may be further refined for defining quality frameworks to be used in different experiences. Each of these models is divided into three parts, namely fundamental concepts, metrics and context. We illustrate our proposal providing a multi-level reference model in the context of collection libraries quality evaluation.