Abstract: The transient processes of pyrene (Py) have been studied by measuring fluores-cence lifetime and nanosecond time-resolved transient absorption spectra in n-hexane, cyclohexane, acetone, and acetonitrile. The single exponential decay for pyrene-hexane and pyrene-oyclohexane systems were observed, yet the double exponential decay were shown for the pyrene-acetone and pyrene-acetonitrile systems (Table 1 of the Chinese text). In the transient absorption spectra, the absorbance of excited triplet pyrene was observed for the pyrene-acetonitrile system (Fig.2 of the Chinese text). This result clearly demonstrate that there are interaction between pyrene and acetonitrile and pyrene cation radical is formed by electron transfer from the excited singlet state of pyrene to acetonitrile. According to experimental results, the photophysical mechanism for the four systems was suggested.