Porous NOTT-202a shows exceptionally high uptake of SO2, 13.6 mmol g(-1) (87.0 wt %) at 268 K and 1.0 bar, representing the highest value reported to date for a framework material. NOTT-202a undergoes a distinct irreversible framework phase transition upon SO2 uptake at 268-283 K to give NOTT-202b which has enhanced stability due to the formation of strong pi...pi interactions between interpenetrated networks.