The cyclization of polystyrene chains ((M) over bar(n) = 2600, (M) over bar(w)/(M) over bar(n) = 1.07; (M) over bar(n) = 4500, (M) over bar(w)/(M) over bar(n) = 1.08), both-ends labeled with 1-pyrenyl groups, was studied in the range of temperatures 25-90 degrees C, in two good solvents (toluene and 1,4-dioxane) and one poor solvent (methylcyclohexane). In methylcyclohexane a two-state Birks kinetics is followed, but in good solvents at high temperatures the contribution of geminate pairs recombination must be considered. Values of the standard enthalpies, Delta H degrees, and entropies, Delta S degrees, for excimer formation were determined from the van＇t Hoff plots. The Delta S degrees values are solvent and chain length dependent, while the Delta H degrees values are, within the experimental error, chain length independent.