Examining the knowledge transfer process in sustainable contexts, we identified various gaps, which we analyzed in this study. First, we compare the temporal context of firms with eco-innovation strategies before and after the financial crisis of the first decade of the 21st century. Second, we analyze the firms’ knowledge transfer, from the use of knowledge sources to innovation through intellectual property. Third, we consider the influence of firm age on firms with eco-innovation goals and the influence of size on intellectual property. We used data from a sample of 3004 firms prepared by the Spanish National Statistics Institute for two different time periods: 2009 and 2014. Our results suggest that firms that achieve sustainable innovations do not show large differences in behavior in the two economic periods. We found that knowledge in firms with eco-innovation goals is transferred through intellectual property. The results show that firm age and size influence these processes during the years analyzed and thus have various implications for theory and for small firms, which are generally family firms. Small and family firms should strengthen their registration of intellectual property so that their knowledge transfer process ends in innovations for both the firm and the market.