Abstract : This work dealt with the study of peptides and oligonucleotides by in-source fragmentation with a Quattro II (Micromass). This instrument was first evaluated in the positive mode with peptide thioesters and then in the negative mode with brominated oligodeoxynucleotides whose fragmentation of the modified base allows its location in the sequence. The fragmentation of peptide acetals and diols (hydrated aldehydes) leads to a same cyclic final ion. The hydration of peptide aldehydes was studied in solution by NMR and in the gas phase by in-source fragmentation. The information coming from this work allowed a better understanding of the mechanism of the ligation chemistry by oxime bond.