We also performed wet-dry cycling 66 in a sealed glass capillary and monitored it by in situ synchrotron PXRD (Figure ). D-U hyd was thermally stable until 410 K, when it started transforming into different crystalline phases (Figure ). Rietveld analysis showed that after heating to 490 K (Figure ), the solid sample was composed of D-U anhyd, D hydrate, 67 and D anhydrate. We also tested if D-U base-pairing occurs in the presence of A. We employed different reaction conditions and demonstrated that D-U base-pairing is selective and occurs with A in the reaction mixture (Figure ). Furthermore, the reaction conditions we tested for base-pairing of equimolar mixtures of D and U include different scenarios of prebiotic water availability, ranging from bulk water to dry conditions. One such scenario on an early Earth involves wet/dry cycling in hydrothermal pools, which are considered to be important for the origin of life. Moreover, some of the reaction conditions that we used might be compatible with meteorite post-accretion events such as aqueous alteration and hydrous metamorphism. This is relevant since both D and U have been detected