Spectrophotometric titrations revealed that stability of the quercetin/double stranded (ds) DNA or double stranded (ds) RNA non-covalent complexes is significantly higher compared to the quercetin/ss-RNA complexes. This observation can easily be correlated with the significantly larger aromatic surface of base pairs compared to single nucleobases, and it is in good agreement with other experimental data pointing toward intercalative binding mode of quercetin. Fluorescence increase of quercetin induced by ds-RNA is significantly stronger than observed for ds-DNA, offering usage of quercetin as the ds-RNA selective fluorescent probe. Also, addition of poly G yielded more than order of magnitude stronger changes in UV/visible and fluorescence spectrum of quercetin compared to the changes upon addition of poly A and poly U revealing possible usage of quercetin as a powerful spectroscopic probe for poly G sequences. Stability and stoichiometry of lanthane(III)/quercetin complexes in physiologically relevant aqueous media was determined. The interactions of (LaQ)(3+) with double stranded DNA and RNA were significantly different compared to the free quercetin, revealing increase of complex stability and thus significant impact of La(III) in binding of (LaQ)(3+) to polynucleotides. Similar results were observed for interactions of (LaQ)(3+) with single stranded RNA.