The paper shows that the post-earnings-announcement drift is stronger for conglomerates, despite conglomerates being larger, more liquid, and more actively researched by investors. We attribute this finding to slower information processing about complex firms and show that the post-earnings-announcement drift is positively related to measures of conglomerate complexity. We also find that the post-earnings-announcement drift is stronger for new conglomerates than it is for existing conglomerates and that investors are most confused about complicated firms that expand from within rather than firms that diversify into new business segments via mergers and acquisitions.