Nongovernmental organisations (NGOs) have come to play a major role in contemporary governance systems, and particularly in the fields of water and the environment. Engagement of civil society is a key aspect of water governance, and rules and norms can shape this engagement. There are many studies discussing the way civil society actors attempt to influence rules and norms [1-3]. However, there is limited research examining how rules and norms affect the behavior of civil society actors [4-6]. This research adapts a new institutional theoretical perspective, and examines how formal and informal rules and norms affect the advocacy strategies of coalitions of NGOs. Comparative case study analysis of NGO coalitions advocating against the Xayaburi hydropower dam planned on the Mekong River in Laos was conducted. The comparison was conducted between the Rivers Coalition in Cambodia and the Vietnam Rivers Network during the planning period of the dam between 2011-2013. Analytical framework was developed in order to analyse how formal and informal rules and norms affect advocacy strategies of two NGO coalitions. The framework is based on Institutional Analysis and Development Framework, developed by scholars of new institutionalism [7]. Data was collected through 72 semi-structured interviews and field observations of a wide range of activities by NGOs in the Mekong region. Data was analysed adopting grounded theory approach, and was triangulated through examining of literature. The main findings of the research is that formal and informal rules and norms interact with each other in creating influence over advocacy strategies. Different patterns of interactions were identified; these are 1) complementary interactions between formal rules, informal rules and norms 2) competing interactions between formal rules, informal rules and norms. The research also identified that actors play important roles in determining the types of interactions. The research identifies barriers and opportunities NGO actors face within the Mekong region, and concludes with two recommendations: 1) modifications to the formal rules which could facilitate further integration of NGOs and civil society actors into decision-making processes of transboundary water governance, particularly of the 1995 Mekong Agreement and 2) use of analytical framework by NGO and civil society actors in identifying windows of opportunities for advocacy strategies. 1. Betsill, M.M. and E. Corell, Introduction to NGO Diplomacy, in NGO Diplomacy: The Influence of Nongovernmental Organizations in International Environmental Negotiations, M.M. Betsill and E. Corell, Editors. 2008, The MIT Press: London. 2. Finnemore, M. and K. Sikkink, International Norm Dynamics and Political Change. International organization, 1998. 52: p. 887-917. 3. Brown, L.D., et al., Globalization, NGOs and multi-sectoral relations. Working Paper No.1. Hauser Center for Nonprofit Organizations and The Kennedy School of Government. Harvard University, 2000. 4. Ho, P. and R.L. Edmonds, eds. China's embedded activism: opportunities and constraints of a social movement. Vol. 30. 2008, Psychology Press. 5. Brinkerhoff, D.W., State-Civil Society Networks for Policy Implementation in Developing Countries Review of Policy Research, 1999. 16(1): p. 123-147. 6. Bryant, R.L., Explaining State-Environmental NGO Relations in the Philippines and Indonesia. Singapore Journal of Tropical Geography, 2001. 22(1): p. 15-37. 7. Ostrom, E., Institutional Rational Choice: An assessment of the institutional analysis and development framework, in Theories of the Policy Process, P.A. Sabatier, Editor 1999, Westview Press: Oxford. p. 35-71.