The quantitative comparison in Fig. (c) shows that significant amounts of CO 2 can be captured across the compositions studied over a duration of 180 minutes. The total amount of captured CO 2 increases with the CO 2 fraction in the feed gas due to a stronger driving force for hydrate formation. With approximately 80 g of captured CO 2 (and approximately 15 g of captured N 2 ) in the run with the 40 mol% feed gas, almost 20 % of the theoretical maximum capacity of 510 g of gas uptake has already been reached. In a similar run, even 154 g of gas are captured by extending the capture period to 500 minutes (see ESI † Fig. ). In both cases, capture rates remain significant towards the end of the capture period. Hence, the limits of CO 2 uptake do not appear to have been reached. Note that the theoretical maximum capacity has been derived assuming a hydration number of 5.75 for the CO 2 in the sI hydrate, 1200 g of water, and pure CO 2 occupancy of all cages. In reality, many of the cages will be either occupied by N 2 or not occupied at all. A more realistic value for uptake capacity is thus significantly smaller, however, still needs to be experimentally determined for the systems and conditions studied in this work. Regardless of that, more than 100 g of captured CO 2 per kilogram of water is strong evidence for capture in the volume of the ice and cannot be attributed to adsorption alone.