We have studied by gravimetric measurements and FTIR spectroscopy the hydration of duplexes and triplexes formed by combinations of dA_n, dT_n, rA_n, and rU_n strands. Results obtained on hydrated films show important differences in their hydration and in the structural transitions which can be induced by varying the water content of the samples. The number of water molecules per nucleotide (w/n) measured at high relative humidity (98% R.H.) is found to be 21 for dA_n.dT_n and 15 for rA_n.rU_n. Addition of a third rU_n strand does not change the number of water molecules per nucleotide: w/n=21 for rU_n*dA_n.dT_n and w/n=15 for rU_n*rA_n.rU_n. On the contrary, the addition of a third dT_n strand changes the water content but in a different way, depending whether the duplex is DNA or RNA. Thus, a loss of four water molecules per nucleotide is measured for dT_n*dA_n.dT_n while an increase of two water molecules per nucleotide is observed for dT_n*rA_n.rU_n. The final hydration is the same for both triplexes (w/n=17). The desorption profiles obtained by gravimetry and FTIR spectroscopy are similar for the rA_n.rU_n duplex and the rU_n*rA_n.rU_n triplex. On the contrary, the desorption profiles of the dA_n.dT_n duplex and the triplexes formed with it (rU_n*dA_n.dT_n and dT_n*dA_n.dT_n) are different from each other. This is correlated with conformational transitions induced by varying the hydration content of the different structures, as shown by FTIR spectroscopy. Modifications of the phosphate group hydration and of the sugar conformation (S to N type repuckering) induced by decrease of the water content are observed in the case of triplexes formed on the dA_n.dT_n duplex.