The last few decades have seen extensive changes in how organisations rely on Information Technology (IT) to account for key aspects of their operations. Understanding accounting as a process through which organisational reality is shaped, Information infrastructures (II) offers a means for analysing the role of IT in organisational change and how IT over time shapes how organisations account for what they are doing. We investigate changes to organisational II at the University of Sydney's Fisher Library from 1963 to 1975, following the introduction of manual automation systems, the use of mainframe computers, and the introduction of minicomputers into the fabric of the organisation. At Fisher Library, II changed two key functions for which the library is accountable for providing information: (1) what items does the library hold? and (2) where is a specific item when it is not on the shelf? We demonstrate that II becomes visible as a thing when it is of interest to organisational change, whereas over time, II sinks into the organisation, becoming a transparent medium that is nonetheless shaping organisational reality. This study uses Fritz Heider's theory of thing and medium to describe how over time IT changes an organisation's account for key aspects of its operation.