One of the main physical effects caused by fire in tough stones is the modification of the pore system through the generation of fissures. The pore size distribution of stones is one of the most important properties influencing stone decay. Pore distribution conditions the absorption and circulation of water within the stone, which has an outstanding influence in other decay agents. These influences should be taken into account in addition to the possible damage generated by the fire itself. The aim of this work has been to characterize the evolution of the porosity of four Hungarian dimension stones at different temperatures. Laboratory simulated burning with oven-based techniques was carried out on three limestones and a rhyolite tuff and samples were analysed by means of Mercury Intrusion Porosity (MIP) and Scanning Electron Microscopy (SEM). Results show the influence of the compactness on the porosity increment after heating. The more compact the stone the greater the porosity increment. When the stone is very compact (e.g. dense limestone), the porosity increases by more than 13 times after heating.