Most of the western tonal music is based on the concept of tonality or key. It is often desirable to know the tonality of a song stored in a symbolic format (digital scores), both for content based management and musicological studies to name just two applications. The majority of the freely available symbolic music is coded in MIDI format. But, unfortunately many MIDI sequences do not contain the proper key meta-event that should be manually inserted at the beginning of the song. In this work, a polyphonic symbolic music representation that uses a tree model for tonality guessing is proposed. It has been compared to other previous methods available obtaining better success rates and lower performance times. KEY WORDS Applications in multimedia, music information retrieval, tonality, cognitive modeling