The objective of this study is to examine the impact of firms' financing constraints on innovation activities in renewable (REN) versus fossil-fuel (FF) technologies. Our empirical methodology relies on the construction of a firm-level dataset for 1,300 European firms over the 1995-2009 period combining balance-sheet information linked with patenting activities in REN and FF technologies. We estimate the importance of the different types of financing (e.g. cash flow, long-term debt, and stock issues) on firms' patenting activities for the different samples of firms. We use count estimation techniques commonly used for models with patent data and control for a large set of firm-specific controls and market developments in REN and FF technologies. We find evidence for a positive impact of internal finance on patenting activities for the sample of firms specialized in REN innovation, while we find no evidence of this link for other firms, such as firms conducting FF innovation or large mixed firms conducting both REN and FF innovation. Hence, financing constraints matter for firms specialized in REN innovation but not for other firms. Our results have important implications for policymaking as the results emphasize that small innovative newcomers in the field of renewable energy are particularly vulnerable to financing constraints.