Summary: A formal method that enables one to design a logic control system from specifications given in natural language is presented. The aim of the proposed method is to prevent a designer’s faults stemming from a misinterpretation of specifications. A significant part of the present paper is devoted to the presentation of the formal framework that underlies this formal design method: algebra II (called I). The operations and relations in this algebra allow one to state formally specifications of a logic control system and to detect inconsistencies in a specifications set. This inconsistency problem is solved by introducing priority levels. From the consistent set of specifications obtained, control laws can be generated by using theorems and properties of algebra II. Application of this formal design method to an industrial example illustrates its main advantages.