The fluorination of K[B(CN)(4)] with CIF is studied by millimolar test reactions in aHF and CH2Cl2 solution and by subsequent identification of intermediates such as B-CF=NCl, B-CF2-NCl2, and B-CF3 species as well as NCl3 by F-19, B-11 NMR, and Raman spectroscopy, respectively. At first one cyano group of K[B(CN)(4)] is converted fast into a CF3 group, and with increasing fluorination the reaction becomes slower and several intermediates could be observed. On the basis of these results, a synthesis was developed for K[B(CF3)(4)] on a 0.2 molar scale by treatment of K[B(CN)(4)] diluted in aHF with CIF. The course of the reactions was followed by (i) monitoring the vapor pressure inside the reactor, (ii) observing the heat dissipation during CIF uptake, and (iii) measuring the volume of the released nitrogen gas. Since the fluorination of the last cyano group proceeds very slowly, the selective synthesis of K[(CF3)(3)BCN] on a 0.2 molar scale is possible, as well. The analysis of the mechanisms, thermodynamics, and kinetics of the fluorination reactions is supported by density functional theory (DFT) calculations.