This paper reports on the qualitative aspect of a study which investigated the importance of interpersonal trust (cognitive trust, affective trust) in virtual communities of developers involved in open source projects at the SourceForge.net web portal. The qualitative responses to an online survey from developers involved in open source projects at SourceForge.net provided rich contextual insights into open source software development. The interpretative analysis of these qualitative data provided a number of interesting findings that show interpersonal trust (cognitive, affective) is important to the effectiveness of virtual communities of core developers involved in open source projects. The findings also show that trust is dynamic and built over time, that there are different levels of trust within an open source project and cognitive and affective trust operate interdependently.