The automatic checking of quality requirements will play a fundamental role in the future market of web services. The reason is that it will allow to build economically-optimal systems whose quality level can be guaranteed. In this paper, we identify some of the main problems with which this kind of futures systems are going to be faced, and also propose a realistic proposal to solve them. The key point is to view quality requirements from a twofold perspective: a natural language sentence and a constraint on a quality attribute. Thanks to this principle, some of the classical disadvantages of formal methods may be overcome.