AbstractKinetic studies of reactions in systems containing polymers have led to the following observations: (1) In dilute solution the thermal and the photochemical isomerization of azobenzene residues is not impeded by their incorporation into a polymer backbone. In the glassy state, the kinetics of the isomerization of azobenzene residues in polymer side chains or backbones reflect the distribution of free volume and volume relaxation. (2) Reactions of polymers with low molecular weight reagents may be affected by neighboring group effects and by the contribution of the polymer backbone to the polarity of its microenvironment. (3) Attack of an enzyme on a polymer side chain depends sharply on the spacing of the susceptible bond from the chain backbone. (4) The expected “kinetic excluded volume effect” in reactions between two flexible chain molecules was not observed. (5) Reactions involving groups attached to a swollen crosslinked gel may reflect spatial variations of the local polarity. (6) Attack of water‐soluble reagents on latex particles may be restricted to the surface of the particle. (7) The presence of a polyion exerts various effects on reactions involving two small ionic sp