The assessment and prediction of marital outcome in couple therapy has intrigued many clinical psychologists. In this work, we analyze the significance of various acoustic features extracted from couples' spoken interaction in predicting the success or failure of their marriage. We also investigate whether speech acoustic features can provide complementary information to behavioral descriptions or codes provided by human experts ( e.g., relationship satisfaction, blame patterns, global negativity). We formulate marital outcome prediction as both binary (improvement vs. no improvement) and multiclass (different levels of improvement) classification problem. Our experiments show that acoustic features can predict marital outcome more accurately than those based on behavioral descriptors provided by human experts. We also find that dialog turn-level acoustic features generally perform better than frame-level signal descriptors. This observation supports the notion that the impact of the behavior of one interlocutor on the other is more important than the behavior itself looked in isolation. Finally, acoustic features together with human-derived behavioral codes show the best performance in outcome prediction, suggesting some complementarity in the information captured by these behavioral representations.