In this paper I use the concepts of musical instrumentation and instrumentality as a way of placing the development of studio-based electroacoustic music in historic context. Histories of mid-century electronic music have often been framed primarily in terms of compositional philosophy: musique concrète, elektronische Musik, etc. Here, however, I highlight a different aspect of the story by casting the development of studio-based electroacoustic music as a radical shift in musical instrumentation. My core argument is that, with the emergence of studio-based electroacoustic music, tape recorders, digital computers, and electronic music studios became musical instruments. This development was paralleled by a radical rethinking of the very concept of musical instrumentality—albeit one that appears not to have gained traction in the wider public imagination (in the sense that tape recorders, computers, and studios are not widely considered ‘musical instruments’). My aims in this paper are: to revisit this instrumental rethinking from the perspectives of some of those involved in designing and making the new instruments in the 1950s and 60s; and to assess what was radical about it by weighing it against the more conventional criteria of musical instrumentality outlined by Hardjowirogo in her article ‘On the Construction of Instrumental Identity.’