The Sustainable Development Goal (SDG) 14.3.1 Indicator calls for the "Average marine acidity (pH) measured at agreed suite of representative sampling stations". The Methodology provides the necessary guidance on how to conduct ocean acidification observations, what to measure and how, providing standard operating procedures and methods approved by the ocean acidification community. It further provides support on what kinds of data to collect, and how to submit, towards the SDG 14.3.1 Indicator to IOC-UNESCO to enable the collection and comparison of ocean acidification data worldwide. The associated data and metadata files enable the collection and submission of the relevant data.