The specification phase is one of the most important and least supported parts of the software development process. SAREL (Assistance System for Writing Software Specifications in Natural Language) has been conceived as a knowledge based tool to improve the specification phase. The purpose of SAREL is to assist engineers in the creation of software specifications written in natural language (NL). The aim of the paper is to present the creation process of the knowledge base and the requirements base corresponding to a software specification written in NL. This process uses some linguistic engineering tools developed in our department designed to process documents that contain preliminary software specifications. These documents are divided into several parts. We can distinguish the Introduction and the Overall Description as parts that should be used by the knowledge base construction. Up to a certain point, these two parts contain all the background information needed to understand the problem as a whole. The information contained in the specific requirements section corresponds to the information represented in the requirements base.