A new study in the United States has concluded that pepper spray can deter bears as well as a firearm.
Thomas Smith, who is in charge of the research at

Brigham Young University, studied 20 years of bear encounters in Alaska.  In cases where humans came into contact with the animal, pepper spray stopped an aggressive bear ninety two percent of the time.  The motivation of the bear for human encounter had no affect on the effectiveness of pepper spray.
However, researchers have warned that spraying pepper spray may also attract bears because of a lingering residue.