In some aggregates used to produce asphalt mixes there were revealed mineral transformations which are typical for acid drainage processes. The main process which was observed in the course of these changes was iron leaching from primary minerals of rocks which are the raw material for production of aggregates. Mineralogical studies allowed also to determine the presence of other, less resistant to weathering, primary minerals which are potential sources of metals such as nickel, copper, zinc and lead. Disclosure of the mechanism of leaching of metals from aggregates used in road construction, in spite of its low intensity, indicates the necessity of more accurate diagnosis of their mineral composition. It is particularly important in case of using them in massive, unbounded layers, potentially available for water, such as base courses.