Ocean acidification poses multiple challenges for coral reefs on molecular to ecological scales, yet previous experimental studies of the impact of projected CO2 concentrations have mostly been done in aquarium systems with corals removed from their natural ecosystem and placed under artificial light and seawater conditions. The Coral–Proto Free Ocean Carbon Enrichment System (CP-FOCE) uses a network of sensors to monitor conditions within each flume and maintain experimental pH as an offset from environmental pH using feedback control on the injection of low pH seawater. Carbonate chemistry conditions maintained in the −0.06 and −0.22 pH offset treatments were significantly different than environmental conditions. The results from this short-term experiment suggest that the CP-FOCE is an important new experimental system to study in situ impacts of ocean acidification on coral reef ecosystems. ; David I. Kline, Lida Teneva, Kenneth Schneider, Thomas Miard, Aaron Chai, Malcolm Marker, Kent Headley, Brad Opdyke, Merinda Nash, Matthew Valetich, Jeremy K. Caves, Bayden D. Russell, Sean D. Connell, Bill J. Kirkwood, Peter Brewer, Edward Peltzer, Jack Silverman, Ken Caldeira, Robert B. Dunbar, Jeffrey R. Koseff, Stephen G. Monismith, B. Greg Mitchell, Sophie Dove & Ove Hoegh-Guldberg