The method of determination of the total ozone content (TOC) from measurements of the differential attenuation of solar radiation by the atmosphere in the near UV spectral range (300-350 nm) is, in fact, a standard method being one of the most common in use. In this case, the aerosol and molecular scattering are taken into account and it is conventionally believed that the ozone exclusively determines the molecular absorption in this spectral range. The investigations carried out in the 1980-1990s at the Institute of Atmospheric Optics SB RAS have shown that there exists a weak absorption by water vapor in this region. The total water content in the atmosphere is large enough to contribute to the measured difference of the atmospheric optical depths and to result in a systematic error. In this case the TOC value, determined without the account of water vapor, may appear to be overestimated. The estimates made in the paper indicate that the correction value can reach tens of percent.