Open Source Software (OSS) is increasingly used in Component-Based Software Development (CBSD) of large software systems. An important issue in CBSD is selection of suitable components. Various OSS selection methods have been proposed, but most of them do not consider the software architecture aspects of OSS products. The Software Architecture (SA) research community refers to a product’s architectural information, such as design decisions and underlying rationale, and used architecture patterns, as Architecture Knowledge (AK). In order to investigate the importance of AK of OSS components in integration, we conducted an exploratory empirical study. Based on in-depth interviews with 12 IT professionals, this paper presents insights into the following questions: 1) what AK of OSS is needed? 2) Why is AK of OSS needed? 3) Is AK of OSS generally available? And 4) what is the relative importance of AK? Based on these new insights, we provide a research agenda to further the research field of software architecture in OSS.