The Ocean Acidification monitoring report presents the research results of the WIOMSA supported ocean acidification projects in Kenya, Mauritius, Mozambique, Seychelles, South Africa and Tanzania. The research provides a baseline that will foster the development of an integrated science strategy for ocean acidification monitoring, research and impact assessment. It presents a review of the current state of knowledge on ocean acidification in the WIO region and identifies the gaps in information required to improve understanding and address the consequences of ocean acidification. The report consists of seven chapters. Chapter 1 introduces the subject of ocean acidification and chapters 2 to 7 summarize the results of ocean acidification monitoring in the six countries that participated in the four-year monitoring project. Lessons learned and recommendations are presented for each country.