Synonyms for Solemn (same or very similar meaning)
WordNet sense 1 (dignified and somber in manner or character and committed to keeping promises):
sedate, sober, grave
WordNet sense 2 (characterized by a firm and humorless belief in the validity of your opinions):
earnest, sincere
WordNet sense 3 (concerned with work or important matters rather than play or trivialities):
serious
From the ODE community, based on WordNet
add/edit