Euchroite, ideally Cu2(AsO4)(OH)·3H2O, is a supergene copper arsenate mineral, known from a number of oxidation zones in a form of emerald-green crystals. Here, we investigated thermodynamics, crystal structure, and parageneses of euchroite from the Svätodušná Cu-deposit near the village of Ľubietová (Slovakia). The actual chemical composition of the studied sample is Cu2[(AsO4)0.98(PO4)0.02](OH)·3H2O. A single-crystal study added the positions of H atoms to the previously known structural model of euchroite. All of the (H2O) groups in the structure are non-transformer, with their O atoms four-fold coordinated. The structural formula of euchroite can be written as [Cu22+(AsO4)(OH)(H2O)3]. The measured thermodynamic properties of Cu2[(AsO4)0.98(PO4)0.02](OH)·3H2O are ΔfH° = −1884.7 ± 4.7 kJ mol−1, S° = 276.6 ± 3.3 J mol−1 K−1, and ΔfG° = −1555.7 ± 4.8 kJ mol−1. The equilibrium constant for the reaction Cu2(AsO4)0.98(PO4)0.02(OH)·3H2O + 3H+ = 2Cu2++0.98H2AsO4−+0.02H2PO4−+4H2O is log Ksp = 4.19 ± 0.87. We estimated the properties of the end-member euchroite Cu2(AsO4)(OH)·3H2O with log Ksp = 4.19. Euchroite is metastable with respect to olivenite, and numerous observations document this metastability. Yet, there are many samples that persist for centuries without change. Observations in the field show that there are also pseudomorphs of cornwallite after euchroite or complex pseudomorphs of cornwallite + cornubite + olivenite after euchroite. Euchroite at the site studied is younger than chalcophyllite and strashimirite; the temporal relationship to parnauite is more complex, as euchroite can be older or younger than parnauite. Our results suggest that euchroite is one of the less-stable copper arsenates, but the energetic margin to the stable phases in the CuO–As2O5–H2O system is small.