Understanding how policymaking processes can influence the rate and direction of socio-technical change towards sustainability is an important, yet underexplored research agenda in the field of sustainability transitions. Some studies have sought to explain how individual policy instruments can influence transitions, and the politics surrounding this process. We argue that such individual policy instruments can cause wider feedback mechanisms that influence not only their own future development, but also other instruments in the same area. Consequently, by extending the scope of analysis to that of a policy mix allows us to account for multiple policy effects on socio-technical change and resultant feedback mechanisms influencing the policy processes that underpin further policy mix change. This paper takes a first step in this regard by combining policy studies and innovation studies literatures to conceptualise the co-evolutionary dynamics of policy mixes and socio-technical systems. We focus on policy processes to help explain how policy mixes influence socio-technical change, and how changes in the socio-technical system also shape the evolution of the policy mix. To do so we draw on insights from the policy feedback literature, and propose a novel conceptual framework. The framework highlights that policy mixes aiming to foster sustainability transitions need to be designed to create incentives for beneficiaries to mobilise further support, while overcoming a number of prevailing challenges which may undermine political support over time. In the paper, we illustrate the framework using the example of the zero carbon homes policy mix in the UK. We conclude with deriving research and policy implications for analysing and designing dynamic policy mixes for sustainability transitions.