The objectives of reusing software are to reduce the cost and amount of resources used to produce quality software that is on time. These objectives are achieved by reusing software artefacts. The reuse insensitive software development approaches, such as component based software development (CBSD) and software product lines (SPL) development, make use of reusable software assets. The use of open source software (OSS) is common in the software industry, especially in CBSD. However, recent research suggests the use of OSS in SPL. In this paper the results of a mixed method study are presented. The study focuses on identifying the factors affecting reusability of software in a reuse intensive software development environment. The first part of the study is based on interviews with experts and professionals working with OSS in a reuse intensive environment. The next part describes a survey is conducted to assess the importance of the factors. The procedures followed and results obtained of the both research activities are presented. © 2011 IEEE.