From experimental PXRD patterns, we found that TMA was thermally stable up to 110 °C (SI, Figure ). In addition, TMA is porous to nitrogen and has a SA BET of 910 m 2 g -1 at 77.3 K (SI, Figure ). However, the PXRD pattern recorded after gas sorption analysis indicated that there was a small amount of the non-porous TMA polymorph in the sample (SI, Figure ). Investigating this further, we found that TMA transforms to TMA at 130°C (SI, Figure ) after 6 hours under dynamic vacuum at room temperature (SI, Figure ), or after standing in humid air after activation (SI, Figure ), indicating that TMA is metastable, as often observed for porous molecular crystals with low densities. This might explain why TMA (1) was not discovered before; conceivably, it might never have been discovered without the guidance provided by CSP. By contrast, we did not find any experimental evidence for predicted form 2; this might be because 1 is more effectively stabilized by solvent, and hence 2 is not competitive.