Actions needed to mitigate and adapt to climate change have often synergies and trade-offs with sectoral and sustainable development priorities, and the recent focus on SDGs and sustainable transitions highlights the need to integrate climate action into other policy spheres. This process is known as climate policy integration (CPI) or climate mainstreaming. Enhancing its understanding as a public policy making process can provide insights for its operationalization, which becomes relevant in the context of the implementation of Nationally Determined Contributions in the global South. This paper aims to enhance the understanding of the how windows of opportunity for CPI are formed by drawing on elements from the multiple streams framework (MSF) developed by John Kingdon. This paper analyses grey literature regarding two cases of climate mainstreaming initiatives implemented by international cooperation organizations. The results show that relevant elements from the MSF, such as, attachment to other high-profile national issues, timing the integration with routine institutional procedures, and the presence of policy entrepreneurs, have been catalysing factors for CPI in the context of such initiatives. However, we can only assess the value of this analytical framework for CPI by testing it systematically through case studies in a variety of contexts.