Activities of fluoride and foluorosilicate in chromium plating baths were studied by using a fluoride electrode, and the following results were obtained.(1) In chromium plating baths, the activity of fluoride was less than that of the additional fluoride. The reason for this was that F-+H+→HF.(2) The activity of fluoride well corresponded with the value calculated from pH of the plating bath and ionization constant of HF, 7.2×10-4.(3) The activity of fluorosilicate was quite the same as that of fluoride at concentrations of less than 10-3mol/l or at higher pH. The reason was that the fluorosilicate was completely decomposed at low concentrations, and it was decomposed to fluoride at higher pH as follows:SiF62-+6OH-→6F-+[Si(OH)6]2-(4) The difference between activities of fluoride and fluorosilicate under other conditions was due to the formation of complex salt by fluorosilicate.