[en] Characteristics of hydrogen (and helium) burning as depending on the density at the envelope temperature are studied. The influence of chemical composition on the behaviour of the flash is investigated. It is shown that if the density at the bottom of the envelope is greater than 10⁶ g/cm³ hydrogen burning almost immediately leads to helium ignition. If 3 x 10⁵<=qsub(c)<=10⁶ then helium begins after the exhaustion of a considerable part of hydrogen. The influence of the hydrogen burning kinetics on the character of the flash is discussed. (author)