The objective of the research study was to assess the effect of different hydrothermal treatment methods (which differed in the volume of the water consumed) on the content of nutrients and bioactive components in the selected varieties of groats and rice available in the Polish market. The highest losses were reported when those raw materials were boiled using a ‘loose-grains’ method; the average dry mass losses were as follows: buckwheat groats: 8.5 %; barley groats: 6.3 % ; rice: 6.3 %. The content of protein was significantly differentiated and depended on the type of the raw material. The highest hydrothermal treatment-dependent decrease in the content of this component was found in the buckwheat groats and the lowest in the rice. Irrespective of the boiling method, the boiling of groats and rice resulted in the significant decrease in the content of lipids and mineral compounds in the form of ashes. The highest decrease in the level of those components was reported in rice (in particular in the de-hulled Basmati rice). The content of dietary fibre was significantly reduced; this was the effect of hydrothermal treatment. As for the buckwheat and barley groats, the reduction reported was, respectively, 55.5 % and 57.1 %, and as for the various rice types: 28.4 % on average. The hydrothermal treatment processes caused the contents of phenolic compounds to significantly decrease, especially those that consumed the highest volumes of water (the so called ‘dense cooking method’); as regards the barley and buckwheat groats, that decrease exceeded 50 % of the initial amount of those components. The buckwheat and barley groats, and among them the BIO buckwheat groats, were characterised by the highest antioxidative capacity of the unprocessed material. In most cases, the methods of culinary treatment processes applied impacted the decrease in the antioxidative activity of the processed groats compared to the raw material.