The CO2 controlled release experiment “Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage” (QICS) assessed the impacts of potential CO2 leakage from sub-seabed carbon capture and storage reservoirs to the marine environment. During QICS, CO2 gas was released into shallow sediment in Ardmucknish Bay, Scotland, in the spring and summer of 2012. As part of this project, we investigated the effects of CO2 leakage on sedimentary phosphorus (P), an essential nutrient for marine productivity. We found no statistically significant effects during QICS, as the solid-phase P content in the sediment was constant before, during, and after exposure to CO2. However, laboratory experiments using marine sediment standard materials as well as QICS sediment revealed substantial differences among these different sediment types in their potential for P release during CO2 exposure. Employing the SEDEX sequential extraction technique to determine the sizes of the major P pools in the sediments, we showed that calcium-bound P can be easily released by CO2 exposure, whereas iron-bound P is a major sink of released P. The overall impacts of CO2 leakage on sediment P behavior appear to be low compared to natural variability.