These notes on the use of one particular form of the time-dependent rate constant to describe the reaction patterns in condensed media have been put together primarily to encourage chemists to try and accept this new way of experimental data treatment. A number of applications is shown and interpretative aspects are discussed. Emphasized are the problems that need to be currently solved. Some of them are also of current interest in condensed phase physics from which the chemical kinetics benefits a great deal. It was inevitable that the choice of subject matter from both rapidly expanding fields and its form of pre- sentation reflect to some extent the author's own interests and some important topics are treated briefly or even omitted. Fully recognizing this, I would like to acknowledge with gratitude the contributions to the subject of all my coworkers in the Laboratories of Lodz, Detroit Mi, MUlheim/Ruhr, and Houston Tx, and of those who helped me in preparing this text. Dr. Wlodzi~ierz Lefik and my son WojciecQ recalculated most of the experimental results, Mrs. Aleksandra Karczewska redrew all the figures. Special thanks go to my wife Ewa for her invaluable assistance in all works and for the final form of the text. AP Lodz, February 1986 CONTENTS 1. Introduction 1 2. Reaction kinetics of species trapped in glassy matrices 6 Excess electrons (6): Post-irradiated decay (6). Spectral relaxation (11). Photostimulated decay (15). Photostimu- lated conversion into trapped hydrogen atoms (17). Radio- luminescence kinetics (21).