The paper focuses on measuring and assessing the relation of adaptive maintenance process and quality of open source software (OSS). A framework for assessing adaptive maintenance process is proposed and applied. The framework consists of six sub- processes. Five OSSs with considerable number of releases have been studied empirically. Their main evolutionary and quality characteristics have been measured. The main results of the study are the following:. 1) Software maintainability is affected mostly by the activities of the 'analysis' maintenance sub-process. 2) Software testability is affected by the activities of all maintenance sub-processes. 3) Software reliability is affected mostly by the activities of the 'design' and 'delivery' maintenance sub- processes. 4) Software complexity is affected mostly by the activities of the 'problem identification', design', 'implementation' and 'test' sub-processes. 5) Software flexibility is affected mostly by the activities of the 'delivery' sub-process.