The present study investigated facial expressions as reactions to positive and negative feedback during a human-computer interaction. Thirty subjects interacted in natural language with a computer system while conducting a mental task, which was designed in the style of a Wizard of Oz scenario. During six different experimental sequences from a well-known memory paradigm (“Concentration”), the wizard produced different feedback as a manipulative tactic to alter the users’ reactions, e.g., praising the subject was assumed to exert a positive influence on the subjects’ moods. To analyze the impact of various feedback on the emotional reaction, facial expressions from all subjects were coded with the Facial Expression Coding System (FACES). In general, the outcomes yielded more negative facial expressions in response to positive feedback as compared with negative feedback. Furthermore, a gender difference was evident: women showed significantly more negative facial expressions in response to praise (“Your performance is improving”) than men. The study and results show that when considering future companion technologies, it is important to carefully investigate users’ reactions to feedback, since paradoxical reactions may ensue.