We study chemical changes in positive photoresist which occur during reclamation processes from various viewpoints. Used photoresist is collected by adding resist solvent and concentrated by evaporation; then its viscosity is adjusted to its original state and it is finally filtered. When the concentration temperature is imposed to the resist, the number of particles slowly increases over time. After about 30 days at a concentration temperature of 40 degree(s)C, the particle number and particle size increase quickly. These particles are formed by coagulation of mainly the tetramers of PAC thermally modified, but not coagulation of mere PACs. Generation of acid is anticipated if the decomposition of PAC or resist solvent happens at the concentration temperature, which deteriorates adhesiveness. Acetic acid, sulfonic acid, carboxylic acid, and formic acid are not formed, however. This result is supported by a patterning experiment.