The goal of this project is to improve the ability of oceanographers to study the marine carbon cycle. This collaboration between institutions in the U.S. and the United Kingdom (UK) combines state of the art modelling and new laboratory measurements of pressure effects on solutions, with expertise in sensor development and ocean pH measurement. The project will make key advances towards (i) quantifying changes in the global carbon cycle from the surface to the interior ocean, (ii) supporting the development of new sensors, and (iii) enabling better integration of autonomous pH measurements into models of the ocean carbon cycle. Beneficiaries will be those of the global carbon observing community requiring accurate calibrations for a wider range of salinities and to deep ocean pressures, and scientists using pH measured on both scales to assemble global datasets, model biogeochemical processes, and ultimately improve decision making. This is a project jointly funded by the U.S. National Science Foundation’s Directorate for Geosciences (NSF/GEO) and the National Environment Research Council (NERC) of the UK via the NSF/GEO-NERC Lead Agency Opportunity. This Lead Agency Opportunity allows a single joint US/UK proposal to be submitted and peer-reviewed by the Agency whose investigator has the largest proportion of the budget. Upon successful joint determination of an award recommendation, each Agency funds the proportion of the budget that supports scientists at institutions in