The paper discusses the issue of the admixtures’ effect on results of granulometric composition determinations by Casagrande’s areometric method in modification by Prószyński. An experiment was conducted involving preparation of samples with known granulometric composition and determination of their composition by means of the aerometric method. Three types of samples were prepared: base (control), containing organic matter, and containing carbonates. Results of the determinations showed a considerable effect of the admixture of organic matter and calcium carbonate on the accordance of determinations, depending on their percent content. The study results also confirmed known dependencies of disturbances in proportions between the sandy and silty-clayey fraction. A practical conclusion from the research is the determination of the threshold of the organic matter and carbonates percent content at which the determination should be absolutely preceded by processes of mineralisation and decalcification.