In studying low dispersion spectra of symbiotic stars and following the usual dereddening procedure we obtain from different pairs of the Balmer lines Hα, Hβ and Hγ considerably different values for the interstellar absorption C at Hβ. There is a tendency the values to be C αβ > C αγ > C βγ. For one of the objects we obtained C αβ = 2.03, C αγ = 1.51, C βγ = 0. The mean values for a sample of 13 objects with measured H γ are C αβ = 1.82, C αγ = 1.63 and C βγ = 1.13. That could be caused by processes taking place in the emitting gas and neglected in the theory as well as by nonlinearity of the detector system. Selfabsorption in the Balmer lines and collisional excitation could affect in different ways the Balmer decrement.