Pyrolysis is a thermochemical decomposition of organic materials conducted under anoxic conditions. An advanced approach represents pyrolysis with microwave heating, which is beneficial due to lower energy consumption and possibly better control of experimental conditions. The typical products of pyrolysis are gases, oil and charcoal or biochar. The industrial use of biochar is well documented [1]. However, more important appeared its application in agriculture and horticulture, because of the positive influence on soil properties and crop productivity, mitigation of green-house gasses [2] and positive effect on plant growth under deficient irrigation conditions [3]. The latter property is ascribed to biochar porosity that is assumed being responsible for holding of a large amount of water and thus regulating soil water management. However, the principle of this effect is still unclear. Biochar is very hydrophobic (contact angle can be as high as 120° [4]), thus the inner surfaces of biochar should not be not easily wettable, which would preventing its hydration and decrease water holding capacity. This work addresses this issue and focuses on the elucidation of the interactions between biochar and water. Biochar was produced by microwave pyrolysis of the municipal sludge catalysed by addition of a small amount of a catalyser. The hydration was studied by a combination of approaches that have already been tested on carbonaceous materials such as humic acids, peats and polysaccharides. The experiments included the determination of non-freezing water (e.g. [5]), desorption enthalpy of water (e.g. [7]) and stability of water clusters (e.g. [6]) present in biochar exposed to different relative humidities. The results revealed very low interaction energies between water and biochar and relatively low amount of non-freezing water. These findings indicate low importance of micropores, higher importance of mezzo and macropores and preferable binding of water by van der Waals forces.