AbstractThe stahilities of triplexes with two sequences, An and (AG)n were studied by 5′‐C2A16C2/5′‐Tn (n = 12, 14, and 16) and 5′‐(AG)8/5′‐(CT)n (n = 4, 5, 6, and 7) systems by variation of the chain length (of oli‐gopyrimidine sequences), the composition of base triad (T A T versus C G C), pH (acidic and neutral), and the concentration of NaCl and MgCl2. No stable C+‐G‐C base triad was formed at neutral pH. However, C+ G C is more stable than TAT in acidic conditions. The triplex formed from the 5′‐C2A16C2 and T12 mixture with 12TAT units has a melting temperature Tm 11 °C. The triplex formed by 5′‐(AG)8/5′‐(CT)4 with four T A T and four C+ G C base triads shows Tm 41 °C. Tm of 5′‐C2A16C2‐[T16]2 triplex increased proportionally to the increased NaCI and MgCl2 concentrations. Tm of 5′‐(AG)8‐ [5′‐(CT)7]2 became constant when NaCI and MgCl2. attained 0.5 M and 5 mM, respectively. The properties of triplexes with An and (AG)n sequences were discussed.