An increasing number of academic studies have been devoted to the organizational adoption of open source software (OSS). Most studies have either focused on determining which reasons influence the adoption of OSS, or which barriers prevent the adoption of OSS. To our knowledge, no prior study has been conducted to determine which barriers exist to the further adoption of OSS in organizations. Studies addressing this issue could provide more insight into whether organizations are effectively able to overcome the initial barriers to adoption, or whether new barriers arise when the organization expands its use of OSS. To this end, we conducted a qualitative field study involving 56 organizations that were asked to report on which barriers existed to their further adoption of OSS. The data was analyzed using a mixed methods approach by combining both qualitative and quantitative techniques. Results showed that the main barrier reported by organizations was a lack of internal and external knowledge. Furthermore, our results indicate that there was no relationship between the barriers reported by organizations and their extent of OSS usage. This indicates that these barriers remain important as organizations increase their assimilation of OSS.