Although the formalisms normally used for describing the semantics of natural languages are far from computationally tractable, it is possible to isolate particular semantic phenomena and interpret them within simpler formal systems. Quantified mass noun phrases is one such part. We describe a simple formal system suitable for the interpretation of quantified mass noun phrases. The main issue of this paper is to develop an algorithm for deciding the validity of sentences in the formal system and hence for deciding the validity of natural language inferences where all the involved noun phrases are quantified mass noun phrases. The decision procedure is based on a tableau calculus.