Recently adopted international environmental treaties on climate change and biodiversity represent some of the most complex agreements ever negotiated, involving science-intensive policy questions and implicating not only governments, but industry and a range of nongovernmental organizations. The inter-connections that should have been taken into account in drafting these agreements were difficult to achieve, given the fractured structure of multilateral institutions. Even if the parties were willing to expose their interests as necessary for effective problem-solving, commitments made to home constituencies made it impossible to be flexible. The Consensus Building Institute has pioneered efforts to design a process that can overcome such barriers in high stakes, high profile, multi-party negotiations. Each has involved senior diplomats in what is best described as collaborative problem solving. This article will use the lessons learned from three experiences to show how parallel informal negotiation provides an alternative –or complement – to more traditional second track diplomacy.