Inorganic mineral fibers of the slag wool and rock wool type and of the glass fiber type were found to exhibit physical disintegration and morphological modifications on the fiber surface when these fibrous products were exposed to environments in which an excessive water medium and heating at temperatures up to 100°C were maintained for a period of several months. The physical disintegration transformed the fine, threadlike fibers into small segments and granules. Scanning electron microscopic examinations showed that test specimens of several slag-rock fibers displayed a rough, eroded fiber surface after the fibers had been immersed in water and heated to temperatures below 60°C for about three months, while similar fibers displayed the development of a reticular network structure, or “honeycomb morphology,” on the fiber surface after they had been immersed in water and heated to temperatures between 85° and 100°C for 14 days. Analyses of the water solution and of the disintegrated fibers suggest that these modifications of the fibrous products are caused by chemical leaching and hydration processes. Thermal conductivity measurements showed that the disintegrated fibers have thermal insulating values about 15% lower than those of normal, threadlike fibers.