Impact investors, seeking financial returns from investments that actively create social or environmental value, use a process known as due diligence to vet incoming investment opportunities. Some industry-wide tools have come to aid investors in this task. Existing metrics are not sufficient to assess the environmental performance of investee companies, the result of which is an allocation of increasing investment dollars into ventures and industries that are not operating within the limits of the socio-ecological systems upon which society depends. This paper proposes the creation of an investor toolkit that would allow users to effectively integrate the principles of strategic sustainable development (SSD) into the due diligence process. Such a toolkit would explain how current due diligence support tools could be used to construct comprehensive sustainability analyses of potential investments. The proposed toolkit was conceived after interviews with impact investors, social entrepreneurs, and industry experts. The research findings confirm a need for investors to use a strategic understanding of sustainability during the due diligence process, in order to increase portfolio value over time.