In this work, we tested the equilibrium constant of hydromagnesite [Mg5(CO3)4(OH)2·4H2O] at 22.5°C proposed by Berninger et al. (2014) against field observations. By field observations we mean that analytical results originate from samples in the natural environment. In our test, we calculated saturation states of hydromagnesite, using the equilibrium constant of hydromagnesite at 22.5°C from Berninger et al., for the carbonate lakes in Qinghai-Xizang Plateau in China and for Salda Lake in Turkey, based on the hydrochemical data from literature. The predictions based on this equilibrium constant indicate that all of these lakes are strongly under-saturated with respect to hydromagnesite. However, hydromagnesite has been shown to form in all of these lakes. Therefore, this equilibrium constant is clearly in direct contradiction with field observations, leading to the conclusion that the equilibrium constant of hydromagnesite of Berninger et al. (2014) is not suitable for geochemical modelling.