Abstract The existence, in the temperature range 20–400°C, of three polymorphic species α, β, and δ of strontium formate, and of four polymorphic species α, β, γ, and δ of calcium formate is established by the means of X-ray diffraction. Crystal lattices of all these varieties are given, and the temperature ranges for their existence have been precisely stated. The importance of the role played by water vapor traces on some polymorphic transitions is pointed out.