Triple helices with G_*G.C and A_*A.T base triplets with third GA strands either parallel or antiparallel with respect to the homologous duplex strand have been formed in presence of Na⁺ or Mg²⁺ counterions. Antiparallel triplexes are more stable and can be obtained even in presence of only monovalent Na⁺ counterions. A biphasic melting has been observed, reflecting third strand separation around 20°C followed by the duplex -> coil transition around 63°C. Parallel triplexes are far less stable than the antiparallel ones. Their formation requires divalent ions and is observed at low temperature and in high concentration conditions. Different FTIR signatures of G_*G.C triplets in parallel and antiparallel triple helices with GA rich third strands have been obtained allowing the identification of such base triplets in triplexes formed by nucleic acids with heterogenous compositions. Only S-type sugars are found in the antiparallel triplex while some N-type sugar conformation is detected in the parallel triplex.