G-quadruplex and i-motif are tetraplex structures present in telomeres and promoter regions of oncogenes. The possibility of producing nanodevices with pH-sensitive functions has triggered the interest for modified oligonucleotides with improved structural properties. We synthesized C-rich oligonucleotides carrying conformationally restricted (2'S)-2'-deoxy-2'-C-methyl-cytidine units. The effect of this modified nucleoside on the stability of intramolecular i-motifs related to vertebrate telomere was investigated by means of spectroscopic methods (UV, CD and NMR). The replacement of selected positions of the C-core by the appropriate C-modified residues induces the formation of stable intercalated tetraplexes at pHs near neutrality. The study demonstrates the possibility of enhancing the stability of i-motif by chemical modifications.