In engineering service-centric systems, it is possible to receive early feedback on candidate services that best match requirements. This includes the possibility of comparing the quality (not just functionality) of candidate services. This paper concentrates on the assessment of service quality at the requirements stage. In doing so, it is found that there is a problem in reaching a common understanding between the parties involved - i.e. different service providers and requirements engineers may use different metrics, units, etc. We present an approach in which our requirements-based service discovery tool exploits an ontology-based quality specification mechanism. This simplifies the problem of reaching a common understanding of quality and allows translation where providers choose to specify quality differently.