Detailed information on research and development (R&D) spending of the private sector is very limited, particularly when the interest is on small and medium enterprises or focuses on companies active in multiple technology areas. This lack of data poses challenges on the robustness of quantitative analyses and, as a consequence, on the reliability of evidences needed, for example, to support policy-makers in policy design. This paper proposes a patent-based method to estimate R&D expenditure in the private sector. This approach is then applied to assess private R&D spending in Europe in the context of climate change mitigation technologies. Findings show that R&D strategies in Europe are both country and technology dependent. Furthermore, R&D budget does not easily flow across national borders, but, when it is allocated to other countries, it is mainly done because of geographic or technological proximity.