Fourier Transform Infrared Spectra of triple stranded polynucleotides containing homopurine dA or rA and homopyrimidine dT or rU strands have been obtained in H20 and D20 solutions as well as in hydrated films at various relative humidities. The spectra are interpreted by comparison with those of double stranded helixes with identical base and sugar composition. The study of the spectral domain corresponding to in-plane double bond stretching vibrations of the bases shows that whatever the initial duplex characterized by a different IR spectrum (A family form poly rA.poly rU, heternomous form poly rA.poly dT, B family form poly dA.poly dT), the triplexes present a similar IR spectrum reflecting similar base interactions. A particular attention is devoted to the 950-800 cm-1 region which contains marker bands of the sugar conformation in the nucleic acids. In solution the existence ofonly N (C3'endo·A family form) type of sugar pucker is detected in poly rU.poly rA.poly rU and poly dT.poly rA.poly rU. On the contrary absorption bands characteristic of both N (C3’ endo-A family form) and S (C2’ endo-B family form) type sugars are detected for poly rU.poly rA.poly dT, poly rU.poly dA.poly dT and poly dT.poly rA.poly dT. Finally mainly S (C2’ endo-B family form) type sugars are observed in poly dT.poly dA.poly dT.