The gelatinization profile was analyzed of aqueous mixtures made from ground instant flakes of four types of cereals and with a varying content of cereal flakes in water. It was found that there were considerable differences in the apparent viscosity of gruels. The following viscosity-differentiating factors were pointed out: origin of starch contained in the flakes, varying contents of starch, hydrothermal treatments and flaking as well as interactions with the main chemical components that formed the matrix on cells of the cereal grain. Linear dependencies were determined between the maximum viscosity of gruels and the content of flakes in the aqueous mixture. Those dependencies make it easier to compare the viscosity of starch gruels or help choose the content level of starch material in food products, in which flakes are to replace starch thickeners. It was found that the gruels made from barley flakes were characterized by the highest viscosity, whereas the barley flakes: by the highest content of β-glucans (3.79 %), a high content of starch (58.3 %), considerable amounts of protein (8.04 %), and a low content of dietary fibre (7.18 %) together with a substantial content of soluble fraction thereof (4.13 %). The wheat flakes had the lowest viscosity during gelatinization, but they contained the highest amount of protein (9.36 %), a high amount of insoluble fibre (7.18 %), and they contained hardly any β-glucans (0.11 %). The gruels made from rye flakes were characterized by a relatively low viscosity despite the fact that the rye flakes contained the highest amount of starch (66.1 %) accompanied by the highest content of fibre (13.09 %), the highest content of insoluble fraction of fibre (8.84 %), but the average content of β-glucans (1.62 %) and the lowest content of protein (4,30 %). The gruels from oat flakes had a relatively low viscosity, and the oat flakes contained: the lowest amount of starch (50.8 %), the highest fat content (6.47 %), a relatively high amount of protein (8.50 %) and β-glucans (3,23 %), and an average amount of fibre (8.20 %). The conditions developed for performing comparative tests on the viscosity of gruels from instant cereal flakes might be applied to design functional food products, in which starch-based thickening agents could be replaced by cereal flakes.