Boron trifluoride, which was photogenerated from benzene diazonium tetrafluoroborate (BDTFB), was analysed quantitatively. When the diazonium compound was decomposed in an insoluble solvent, the residual reactant and decomposition products containing benzene ring could be determined by UV spectrophotometry. But BF₃ was not able to be detected directly because of overlapping of the characteristic band with those of other products. So BF₃ was determined indirectly from spectral change by coordination to lone pair electrons of a dimethylamino compound. When the decomposition was carried out in the presence of p-(N, N-dimethylamino)- fluorobenzene in a solution of isopropanol or dioxane, 1.0 mole of BF₃ was determined from 1.0 mole of the diazonium compound. Then, the storage stability of BDTFB was investigated by keeping it as a dispersion in IPA or dioxane at 20°C. It was found that BDTFB in dioxane completely decomposed in 3 weeks, and that 60% of BF₃ to have been generated from BDTFB was detected in the decomposition products. On the other hand, no BF₃ was detected in IPA after the complete thermal decomposition of BDTFB which took place in 2 weeks. This analytical method by the use of the spectral change of a dimethylamino compound detects selectively the BF₃ which is useful as the cationic initiator.