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Partial hydration of organic compounds can be achieved by high-pressure crystallization. This has been demonstrated for the high-nitro-gen-content compound 6-chloro-1,2,3,4-tetrazolo[1,5- b ]pyridazine (C 4 H 2 N 5 Cl), which becomes partly hydrated by isochoric crystallizations below 0.15 GPa. This hydrate, C 4 H 2 N 5 Cl· x H 2 O, is isostructural with the ambient-pressure phase α of C 4 H 2 N 5 Cl, but the crystal volume is somewhat larger than that of the anhydrate. At 0.20 GPa, the α-C 4 H 2 N 5 Cl anhydrate phase transforms abruptly into a new higher-symmetry phase, α'; the transformation is clearly visible due to a strong contraction of the crystals. The hydrate α-C 4 H 2 N 5 Cl· x H 2 O can also be isothermally compressed up to 0.30 GPa before transforming to the α'-C 4 H 2 N 5 Cl· x H 2 O phase. The isochoric recrystallization of C 4 H 2 N 5 Cl above 0.18 GPa yields a new anhydrous phase β, which, on releasing pressure, transforms back to the α phase below 0.15 GPa. The structural transition from the α to the β phase is destructive for the single crystal and involves a large volume drop and significant elongation of all the shortest intermolecular distances which are the CH⋯N and CH⋯Cl hydrogen bonds, as well as the N⋯N contacts. The α-to-α' phase transition increases the crystal symmetry in the subgroup relation; however, there are no structural nor symmetry relations between phases α and β., (© Anna Olejniczak et al. 2022.)