When white chocolates are kept molten in storage tanks, problems can arise due to uncontrolled thickening and solidifying of the chocolate mass. The thickening of molten white chocolate was simulated on a laboratory scale using a rotational rheometer under static conditions, interrupted by short shear periods to measure the increasing viscosity. Several chocolates having different dairy components and fat contents were investigated for their tendency to thicken. In addition, sorption isotherms for white chocolates were obtained using dynamic vapour sorption at different temperatures. The sorption isotherms showed the presence of amorphous lactose in all the chocolates that were manufactured from milk powders. Moisture that is released during the crystallization of amorphous lactose causes stickiness and agglomeration of the neighbouring particles and starts the thickening process. This process is highly temperature-dependent. On elevating the temperature the lactose crystallization occurs at lower relative humidities. In order to reduce the tendency of white chocolate to thicken, a high free-fat level should be maintained, based on a high total fat content and on the use of high free-fat milk powders, preferably roller-dried whole milk powders or the combination of skimmed milk powder and anhydrous milk fat.