The photochemical reaction process of anthraquinone-2-sodium sulfonate （AQS） in the mixture of water （H2O） and N-butylpyridinium tetrafluoroborate （[BPy] [BF4]） was studied using the laser flash photolysis technique. Experimental results show that the excited triplet of AQS （3AQS＊） could react rapidly with H2O and the transient absorption spectra greatly changed by increasing the volume fraction of the ionic liquid （VIL） in [BPy][BF4]/H2O mixtures. The absorbance at 510 nm increased gradually with increasing VIL when 0〈VIL〈0.1. By contrast, the absorbance decreased gradually when VIL〉0.1. Otherwise, the absorbance of the band near 380 nm steadily increased. The apparent kinetic parameters of transient species B and ^3AQS＊ are obtained approximately. 3AQS＊ abstracting hydrogen from [BPy]＋ was also explored. It was deduced that the 350-420 nm band was the superposition of the peaks of 3AQS＊ and AQSH＇. The two reactions of 3AQS＊ with [BPy][BF4] and H2O are a pair of competitive reactions. We also concluded that the entire reaction processes slow down in the case of high [BPy] [BF4] concentrations.
