In my experiment, different polyisobutylene derivatives were studied. These derivatives are the following: olefin telechelic, chloride telechelic, hydroxyl telechelic and chloride terminated polyisbulylene (PIB). Atmospheric pressure photoionization (APPI) was applied to characterize and study the ionization of the PIBs as the function of the temperature (250-450) °C. In these experiments, I observed that increasing the temperatures the determined average molecular weights mostly increase.