Among the other methods of water composition diagnostics, laser fluorometry method distimguished with a simple implementation and high sensitivity. The method is based on the principle of calibration of fluorescence signal (FL) of a dissolved substance on the internal benchmark - Raman signal (RS) of light by molecules (water) solvent.   The normalization to the signal RS puts FL on a quantitative basis. The laser probing method is practically the only active method for remote and non-contact analysis of the water mass composition.   PL and Raman signals will be comparable with the impurity concentrations of 10-3 - 10-6mkg·L-1. Vibrational spectra of H2O molecules responsible for the RS, have a relaxation time ~ 10-13s, so Raman signals are linearly related to the intensity of the exciting radiation within a wide range.   To calibrate fluorescence weak organic substances (OS) bands in purified water Stokes component of water RS with a frequency ν = 3440 cm-1 is usually used. In the present paper PL spectra for the samples from municipal water systems at different times of the year (Fig.1) and various cleaning methods have been investigated with this method. We have also investigated water quality after its purification by various domestic filter. We have found that one of the effective and most accessible methods of water purification is its freezing. Examining spectra of water purified by freezing has shown that the intensity of the peak related to the natural dissolved organic matter (DOM) falls 2 times. The same effect (natural freezing out of DOM) can explain water quality improving during winter period. The information obtained may contribute to the identification of the dissolved impurities in the complex spectrum of the fluorescence of natural aqueous solutions. By the intensity of bands in the PL spectra one can determine the impurity concentrations up to 10-7-10-8 mg·L-1. This value is lower than that of many MACs of dissolved OS.