The storage properties of fuels are critical in regard to the introduction of a new fuel into markets. The fuel must be homogeneous, and the properties of the fuel should not change significantly during the storage at the customer's facility. In this research, the storage stability of wood-based pyrolysis liquids was followed by analysis of the changes in the physical properties and chemical composition during storage. The main physicochemical changes took place during the first six months. The high-molecular-mass (HMM) fraction of water-insolubles, which were originally lignin-derived material, increased, because of polymerization and condensation reactions of carbohydrate constituents, aldehydes, and ketones. Therefore, the average molecular mass of pyrolysis liquids increased, which was also observed as an increase in viscosity. There was a clear correlation of the average molecular mass with the viscosity, water-insolubles, and the HMM fraction of water-insolubles. The chemical changes in the aging were similar to those which occurred during the accelerated aging test. The decrease in volatile aldehydes and ketones increased the flash point of the liquids. The increase in viscosity increased the pour point. Water was formed as a byproduct in various condensation reactions. Increases in the amount of water decreased the heating value. The density of the liquid increased, because the increase in HMM lignin fraction was more significant than the increase in water.