In this report, we combine thermogravimetric analysis, X-ray diffraction and infrared spectroscopy to investigate the chemical and structural evolution when exposed to ambient atmosphere of alkali titanates Rb2Ti2O5, K2Ti2O5, and mixed compound Rb2-2xK2xTi2O5, with 0 ≤ x ≤ 1. We show that a complete solid solution exists between the potassium and rubidium end-members and that mixed compounds exhibit an intermediate behavior. When exposed to the ambient atmosphere, all compounds degrade, with the progressive formation of first hydrates and then bicarbonates and a much quicker degradation of the rubidium titanate as compared to the potassium one. Reversely, we show that when hydrated in the absence of CO2, the crystal structure of K2Ti2O5 is more quickly impacted than that of Rb2Ti2O5. These observations should be useful to understand the evolution of the functional properties of M2Ti2O5 alkali titanates upon ageing in ambient atmosphere, as well as for the optimization of their hydration-dependent properties.