Photochemical and photophysical primary processes of solid poly(ethylene terephthalate) powder were investigated with nanosecond diffuse reflectance laser photolysis and conventional steady-state luminescence techniques. The existence of four transient species was confirmed. To assign these transients, dependencies of the transient absorption spectra and decay kinetics on foreign gas, dopant aromatic compounds which act as a photosensitizer or quencher, and crystallinity were investigated. In addition, the temperature effect was examined. As a result, the transient species, observed only under an aerated condition, was assigned to the precursor species leading to the photooxidation or photodegradation reaction. Other transient species were assigned to three different triplet states, which correspond to three different local structures: monomer site, dimer site, and a site which forms an excimer in the excited singlet state. The generation of three different triplet states is ascribed to an inhomogeneity of the aggregation and relative orientation of the monomer unit. It is discussed that crystalline and amorphous regions, and their boundary region, have a key role in photochemical and photophysical processes in polymer powder.