Recently, automakers have invested heavily in alternative fuel technologies to reduce carbon dioxide (CO2) and nitrogen oxide (NOx) emissions from vehicles. Yet we know comparatively little about how automakers work collaboratively with their suppliers to develop interorganizational eco-innovations. We address this research gap and contribute to the eco-innovation literature by investigating whether suppliers' eco-innovation capabilities in electric, hybrid, and fuel cell technologies influence the degree of eco-innovation co-patenting between Toyota and its suppliers. We also expand research in the strategic alliance literature by exploring the moderating effects of alliance partner diversity. To test our research hypotheses we use data from firms in the Toyota supplier association together with negative binomial and zero-inflated negative binomial regression models. Our empirical results demonstrate that supplier electric capabilities and supplier hybrid capabilities are positively related to the occurrence of eco-innovation co-patents. However, supplier fuel cell capabilities have no effect on eco-innovation co-patenting, especially as firms often choose to develop this technology in-house. Moreover, alliance partner diversity has a negative moderating effect on the relationships between supplier electric capabilities, supplier hybrid capabilities, and eco-innovation copatents. Finally, we evaluate how our findings from the Toyota supplier association compare to similar Japanese automakers and their supplier networks.