This thesis looks at how sound and visuals may be linked in a musical instrument, with a view to creating such an instrument. Though it appears to be an area of significant interest, at the time of writing there is very little existing - written, or theoretical - research available in this domain. Therefore, based on Michel Chion’s notion of synchresis in film, the concept of a fused, inseparable audiovisual material is presented. The thesis then looks at how such a material may be created and manipulated in a performance situation. A software environment named Heilan was developed in order to provide a base for experimenting with different approaches to the creation of audiovisual instruments. The software and a number of experimental instruments are discussed prior to a discussion and evaluation of the final ‘Ashitaka’ instrument. This instrument represents the culmination of the work carried out for this thesis, and is intended as a first step in identifying the issues and complications involved in the creation of such an instrument.