Preliminary research aiming at the reconstitution of concrete and mortar substrate composition with the application of classical analytical methods and XRF is described. Approximate calculations of the substrate composition have been performed on the basis of an algorithm that consists in determining content proportions of calcium oxide, silica, alumina, iron oxide, sulfur trioxide and chlorides in substrates and products, elaborating linear equations with material balance for individual oxides in substrates and products, setting equation systems of two unknowns (cement, sand) for mortars and three unknowns (cement sand, coarse aggregate) for concretes, and finally solving all possible arrangements of equations with the use of a computer software. Mean values of all possible solutions have been accepted as final results. The determined initial composition is close to the real one with maximum differences of about 3%. The elaborated algorithm cannot be applied to calculate substrate composition when sulfur oxides and aluminum occur simultaneously in the balance equation.