The specification phase is one of the most important and least supported parts of the software development process. The SAREL system has been conceived as a knowledge-based tool to improve the specification phase. The purpose of SAREL (Assistance System for Writing Software Specifications in Natural Language) is to assist engineers in the creation of software specifications written in Natural Language (NL). These documents are divided into several parts. We can distinguish the Introduction and the Overall Description as parts that should be used in the Knowledge Base construction. The information contained in the Specific Requirements Section corresponds to the information represented in the Requirements Base. In order to obtain high-quality software requirements specification the writing norms that define the linguistic restrictions required and the software engineering constraints related to the quality factors have been taken into account. One of the controls performed is the lexical analysis that verifies the words belong to the application domain lexicon which consists of the Required and the Extended lexicon. In this sense a synonym management process is needed in order to get a quality software specification. The aim of this paper is to present the synonym management process performed during the Knowledge Base construction. Such process makes use of the Spanish Wordnet developed inside the Eurowordnet project. This process generates both the Required lexicon and the Extended lexicon that will be used during the Requirements Base construction.