In many chemical engineering systems, the classification of product quality characteristics is performed by human experts, due to the absence of measuring devices. The development of mathematical models for such systems is a rather difficult task, since no equations based on first principles can be written. It remains to use black-box modeling techniques, where the qualitative and subjective classifications given by the experts, is the only available information for the output quality variables. In this paper, we propose a fuzzy classifier, which can be used as an adequate and reliable expert system to perform quality classifications. The methodology performs remarkably well in two different cases, showing that it has certain advantages over other black-box modeling techniques.