The monograph is devoted to one of the important sections of modern nuclear chemistry - the use of positron annihilation to study the physical and chemical properties of a substance. Describes the general properties of positrons, the picture of their decelerationin the substance, the processes of formation of the same death of the simplest hydrogen-like "atom" - positronium The ways of using the annihilation of positrons for the study of elementary chemical processes are analyzed: mechanism of electron deceleration, ionization and excitation of molecules, kinetics of reactions of hydrogen-like atoms in gas and condensed phases, as well as for registration of free radicals and for the study of electron pulse distribution. The book is of interest to a wide range of nuclear physicists and physico-chemists.