The accurate time-resolved mass fluxes of individual species further allow us to estimate the composition of the gas captured in the hydrate. In absence of other sinks for CO 2 or N 2 , the steady state composition of the flow being captured -readily computed from the difference between inlet and outlet flow of the individual species -is identical to the steady state composition x hyd,ss of the mixed hydrate (cf. Fig. in ESI †). As is shown in Fig. (c), x hyd,ss ranges from 28.2 mol% (flue gas with 5.0 mol% of CO 2 ) to 82.4 mol% (flue gas with 40.0 mol% of CO 2 ) and increases steadily with increasing mole fraction of CO 2 in the feed gas. These values are smaller than the maximum fractions of CO 2 measured at the reactor outlet at the end of the regeneration period but lie well above those at the theoretical phase envelope calculated using CSMGem for the respective conditions. The same observations are made in a comparative experiment using a single feed gas with 15 mol% of CO 2 but varying pressures (cf. Fig. in the ESI †). Clearly, the majority of the hydrate forms in contact with a gas whose composition contains more CO 2 than the equilibrium concentration on the V-I-H coexistence curve at the given pressure and temperature. Consequently, the CO 2 fraction at the phase envelope in many cases significantly underestimates that of the gas in the hydrate and is therefore of limited usefulness for the planning and modeling of the HBCC process. Instead, the planning and modeling would be more accurately based on the ternary phase diagram of H 2 O-N 2 -CO 2 and other systems relevant for gas separation. Unfortunately, to our knowledge, no such diagrams have yet been published.