Reversed-phase high-performance liquid chromatography (RP-HPLC) was used for rutin (quercetin-3-rutinoside) determination in selected buckwheat products (whole meal flour, broken seeds, seed hulls, herbs and baked cereal breads). The effect of various thermal procedures on content of rutin was evaluated using calcium as an internal tracker to correct changes in mass and composition of the buckwheat products. These factors are very seldom taken into account. The results show non-significant changes in rutin levels obtained in whole meal flour and broken seed samples after thermal treatment up to 150°C. Higher temperature already caused sudden fall in the observed rutin concentrations. The evaporation of some volatile compounds and degradation products can decrease the mass of the samples and formally increase the content of rutin (35.5 ±4.7 mg per 100 g for whole meal flour and 10.2 ±0.4 mg per100 g for broken seeds at 150°C). Serious decrease of rutin contents at elevated temperatures (>150°C) can be explained by its degradation (by breaking off the C-C bond in quercetin-3-rutinoside moiety) and/or evaporation (24.3 ±1.4 mg per 100 g for whole meal flour and 3.06 ±0.3 mg per100 g for broken seeds at 180°C). In case of baked cereal breads the level of rutin changed in dependence on the ratio between buckwheat and corn flour. Longer time leaching and higher temperature implicate higher rutin content in infusions prepared from buckwheat seed hulls and herbs.