<p>Mudstone swelling capacity is an important factor that determines the quality of engineering constructions, in particular, roads, tunnels, dams, etc. The most important parameter is the content of water that determines the stability/behavior of mudstone under different conditions, e.g., under variable temperature and humidity. Laboratory thermal treatment is a widely used method to measure the content of water in mudstone, which controls its swelling capacity. Exploring the swelling properties of samples treated at different temperatures will help to further understand the impact of water content on swelling properties. In this paper, we present new results on the heat treatment of red-bed mudstones of the Shaximiao Formation exposed in the eastern Sichuan Province of SW China in order to trace variations of mass, pore content, microstructural patterns and swelling characteristics. The swelling was traced by scanning electron microscopy and pore tester. The heat treatment changes the microstructure and pore characteristics of the mudstone and its swelling properties. The mudstones yielded three types of water loss patterns caused by the presence of free, absorbed and constitutional waters. The swelling properties appeared to depend on the pattern of the loss of three water types at different temperatures. The increasing temperature resulted in faster dehydration of clay minerals, stronger damage of original textures and microstructures (pore pattern) and stronger swelling. Our results provide a new guideline for selecting a temperature of heat treatment and controlling the content of water during red-bed swelling experiments.</p>