In the context of adsorptive water treatment, exhaust gas treatment, gas storage, or catalysis, porous boron nitride is gaining in scientific attention due to its versatile properties. The material is above all characterized by its high chemical and thermal stability. However, it has recently been shown that porous boron nitride is highly
water-susceptible, which severely limits its application in non-inert atmospheres. So far, solely the influence of chemical composition on water stability has been investigated in literature. In the present work, the influence of different pore sizes is considered. Two different boron nitrides, a mesoporous and a micro-mesoporous one, are investigated with regard to their stability in air, water vapor, and liquid water for multiple weeks. Using different analytical methods, it can be shown that the stability of porous boron nitride strongly depends on pore size and structure. We demonstrate that the mesoporous boron nitride synthesized in this work has much higher water stability than micro-mesoporous boron nitrides. The mesoporous material showed minor to no changes in its chemical structure and isothermal shape when exposed to water vapor and air. We suggest that a higher amount of crystalline hexagonal boron nitride leads to increased stability. This conclusion opens pathways to improve the water stability of porous boron nitride in order to make it suitable for non-inert gas phase adsorption applications.