An expert system, EXPRES (EXPert system for chemical REaction cycles Synthesis), is developed. This expert system can generate two kinds of reaction cycles systematically, i.e. chemical reaction cycles and chemical reaction clusters, with the help of reaction databases and various kinds of knowledge bases. In the reaction data base, known reactions that have already been proven to proceed at reasonable rates are collected. EXPRES has distinctive features such as reaction rules abstracted and evaluated by experts, learning effect and hierarchically structured knowledge base on effective processing and maintenance. The method for conversion of a chemical reaction formula into a list expession in LISP language is also established. EXPRES can be applied to : (1) derive multistep reaction clusters from a single reaction that is difficult to proceed under industrial conditions; or (2) to find chemical heat pump cycles that work within desired temperature range, for effective use of resources or thermal energy.