Oligodeoxynucleotides which include stretches of guanines form a well-known tetrameric structure. We show that the recording of reversible absorbance changes at 295 nm allows to precisely monitor intramolecular guanine (G)-quartet formation and dissociation. Accurate Tm and thermodynamic values could be easily extracted from the data, whereas classical recordings at 260 nm led to a much larger uncertainty and in extreme cases, to completely inaccurate measurements. This inverted denaturation profile was observed for all G-quartet-forming oligonucleotides studied so far. This technique is very useful in all cases where intramolecular or intermolecular quadruplex formation is suspected.