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Aluminum trimesate-based MOF (MIL-96-(Al)) has attracted intense attention due to its high chemical stability and strong CO 2 adsorption capacity. In this study, CO 2 capture and selectivity of MIL-96-Al was further improved by the coordination of the second metal Ca. To this end, a series of MIL-96(Al)-Ca were hydrothermally synthesised by a one-pot method, varying the molar ratio of Ca 2+ /Al 3+ . It is shown that the variation of Ca 2+ /Al 3+ ratio results in significant changes in crystal shape and size. The shape varies from the hexagonal rods capped in the ends by a hexagonal pyramid in MIL-96(Al) without Ca to the thin hexagonal disks in MIL-96(Al)-Ca4 (the highest Ca content). Adsorption studies reveal that the CO 2 adsorption on MIL-96(Al)-Ca1 and MIL-96(Al)-Ca2 at pressures up to 950 kPa is vastly improved due to the enhanced pore volumes compared to MIL-96(Al). The CO 2 uptake on these materials measured in the above sequence is 10.22, 9.38 and 8.09 mmol g -1 , respectively. However, the CO 2 uptake reduces to 5.26 mmol g -1 on MIL-96(Al)-Ca4. Compared with MIL-96(Al)-Ca1, the N 2 adsorption in MIL-96(Al)-Ca4 is significantly reduced by 90% at similar operational conditions. At 100 and 28.8 kPa, the selectivity of MIL-96(Al)-Ca4 to CO 2 /N 2 reaches up to 67 and 841.42, respectively, which is equivalent to 5 and 26 times the selectivity of MIL-96(Al). The present findings highlight that MIL-96(Al) with second metal Ca coordination is a potential candidate as an alternative CO 2 adsorbent for practical applications., Competing Interests: The authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)