Nontypeable Haemophilus influenzae (NTHi) is an important cause of otitis media and respiratory tract infections. Its lipopolysaccharide (LPS) molecule, an outer membrane component, is a major virulence factor of NTHi. The LPS molecule may also be an efficient target for antibodies which might be protective against NTHi disease. The present thesis describes the structural analyses of the LPS from a number of NTHi clinical isolates. These isolates have been selected to be representative of the genetic diversity in the natural population of NTHi. The studies revealed extended inter- and intrastrain variability in LPS expression but also the presence of a conserved structural element. Several novel structural features were found, including epitopes not previously observed in Haemophilus influenzae LPS. The studies were performed using nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry and different chemical degradations of the LPS as the principal methods.