In a study on experience-based learning in serious games, 45 players were tested for topic comprehension by a questionnaire administered before and after playing the single-player serious game Peacemaker (Impact Games 2007). Players were divided into two activity conditions: 20 played a 1-h game with a 3-min half-time break to complete an affect self-report form while 25 also participated in a 20-min reflective group discussion during their half-time break. During the discussion, they were asked by an experimenter to reflect on a set of topics related to the game. We present the analysis of the questionnaires, which illustrates that contrary to our expectations the reflection period had a negative effect on the learning of the players as judged by their performance on closed-form questions at levels 1-5 (out of 6) on the Bloom taxonomy of learning outcomes. The questionnaire also included a few open questions which gave the players a possibility to display deep (level 6) learning. The players did not differ significantly between conditions regarding the questions measuring deep learning. (Contains 3 notes, 4 tables, and 6 figures.)