Thermal stability, structural evolution pathways, and phase transition mechanisms of the calcium oxalates whewellite (CaC ₂O ₄·H ₂O), weddellite (CaC ₂O ₄·(2+x)H ₂O), and caoxite (CaC ₂O ₄·3H ₂O) have been analyzed using single crystal and powder X-ray diffraction (XRD). During single crystal XRD heating experiments, α-CaC ₂O ₄ and the novel calcium oxalate monohydrate have been obtained and structurally characterized for the first time. The highest thermal expansion of these compounds is observed along the direction of the hydrogen bonds, whereas the lowest expansion and even contraction of the structures occur due to the displacement of neighbor layered complexes toward each other and to an orthogonalization of the monoclinic angles. Within the calcium oxalate family, whewellite should be regarded as the most stable crystalline phase at ambient conditions. Weddellite and caoxite transform to whewellite during dehydration-driven phase transition promoted by time and/or heating.