Annealing microcrstalline phenyl-C-61-butyric acid methyl ester (PCBM) above 100 degrees C results in the formation of hole traps. This is observed as an irreversible decrease in the lifetime of the transient conductivity of pulse-ionized microcrystalline PCBM powder at room temperature after annealing at different elevated temperatures (see figure). Caution is therefore advised in the routine use of pre-annealing to "improve" the performance of PCBM-based photovoltaic or field-effect transistor devices.