The main goal of this research is to develop an expert system to help the chemists in determining the structure(s) of unknown compounds, named flavones, from their mass and nuclear magnetic resonance spectra anaylsis, a prototype expert system named Semi Dendral Expert System (SDES) has been designed and implemented to do this job. SDES has the ability to give chemical formula and additional number of OH and OCH3 functional groups that an unknown flavone contains, and has the ability to generate all the possible irredundant structures and predict mass spectrum for each generated structure. Other features of SDES include explanation facilities (WHY and HOW) and interact with the user to an easy use. Also the knowledge base of the system can be changed by the expert. SDES is a rule-based system that uses a forward chaining approach, the system has been implemented using Turbo Prolog version 2.0 programming language on an IBM/PC compatible.