This paper identifies mechanisms that affected over 200 Information System Process Innovation (ISPI) unlearning decisions in three organisational environments over a period that spanned four decades. The analysis is based on previous unlearning studies. Four distinct generations analysed are early computing (1954-1965); main frame era (1965-1983); office computing era (1983-1991), and distributed applications era (1991-1997). These follow roughly Friedman’s and Cornford’s categorisation of IS development eras. We also distinguish four types of ISPI’s: base line technologies, development tools, description methods, and managerial process innovations. Our analysis shows that the most important unlearning mechanisms were new knowledge creation, poor performance, react to changing environment, changes in management, and too complicated to use. In the organisations the unlearning mechanisms varied significantly according to the ISPI category, and time generation. The variation can be thus partly explained by the fact that the technological development and the rapid diffusion of microcomputers in the beginning of 1980s changed IS development (ISD) work, and new skills and ISPIs were needed. In the beginning of 1990s technological platforms, operating systems, databases, tools, and working procedures changed to object orientation, and the previous ISPIs had to be unlearned.