Lately we have seen a growing interest from both public and private organisations to adopt OpenSource Software (OSS), not only for a few, specific applications but also on a more general levelthroughout the organisation. As a consequence, the organisations' decisions on adoption of OSS arebecoming increasingly more important and complex. We present three perspectives organisations canemploy in their decisions: seeing OSS acquisition as a business case, as COTS acquisition, and as architecturalchange within a governance framework. We present case studies of decisions on OSSadoption, and categorise the decision criteria we have found. Our results indicate that for large-scaleadoption of OSS, focus will be on architectural considerations: enterprise-wide architectures will atfirst be a barrier, but in the long term OSS's support of open standards can be a major enabler forOSS adoption. In contrast, in smaller organisations and in small-scale adoption of OSS, the cheapprice of OSS is a major enabler, as it provides a good opportunity for experiments and short-termeconomic benefits. For small organisations these experiments can lead to development of a commonIT-architecture, and in larger organisations OSS can be adopted in niche-areas, without significantlyviolating an existing IT-architecture.Keywords: open source, COTS, IT architecture, governance