There is no satisfactory account of the psychological processes that mediate a news framing effect. Based on an experimental study (N = 1,537), this article presents a mediation analysis of a news framing effect on opinion, testing for two important mediation processes: belief importance and belief content change. Results show that framing is mediated by both belief importance and belief content, with belief content being the more prominent variable. The extent to which each process takes effect depends on a person's level of political knowledge. Knowledgeable individuals are affected to a greater extent via both belief content and belief importance change.