A technique was developed and the optimal parameters were selected for solving the inverse problem of determining the total content (TC) of R-11 freon (CCl3F) from ground-based spectrometric measurements of solar radiation at the St. Petersburg NDACC station using an IFS 125HR Fourier interferometer and SFIT4 software. The main features of the technique were the spectral interval of 830-860 cm(-1) and the previously measured instrument spectral sensitivity function. Continuum absorption of radiation by water vapor and attenuation of radiation by the fi lm of amorphous ice deposited on the receiver were also taken into account. The spectral baseline was corrected by a second-order polynomial and the vertical distribution of water vapor was refi ned to compensate for the variability of the ice-film thickness during solution of the inverse problem. The technique was used to determine the TC of R-11 during 2009-2019. The estimated measurement errors were average systematic error 7.4%; random, 2.9% for the entire observation period. The longterm trend of the TC was assessed as -0.29 +/- 0.07%/yr; mean volume concentration, -0.31 +/- 0.07%/year.