Optimum parameters were selected for determining the total content (TC) of Freon R-12 (CCl2F2) from ground spectrometric measurements of solar radiation at the St. Petersburg NDACC station on a Bruker FS125HR Fourier interferometer. On the basis of the obtained optimum set of parameters the total content of R-12 over the station was established over the period of 2009-2018 with a systematic error of 2.7% and a random error of 2.5% over the whole period of the investigations. The preliminary trend estimate was -0.46 +/- 0.06% per year.