A new perspective on chemistry learning could be the optimal learning moment of view based on flow theory. Optimal learning moments (OLMs) that occur when students experience high levels of interest, challenge, and skill at the same time were investigated in this study. Participants of the study were 47 pre-service science teachers. Data were collected via the experience sampling method in a university-level chemistry course which was given online due to the COVID19 pandemic, and an online form was used to collect data at specified points of the course. It was found that participants reported OLMs 23.5% of the time. Also, the occurrence of OLMs in different units (content) or different parts (activities) of the course was investigated. There was no significant difference in OLMs at different units of the course. However, there was a significant difference between the course's lecturing, mini-testing, and problem-solving parts. Optimal learning moments occurred in mini-testing and problem-solving parts more than in the lecturing part of the course. Thus, it can be concluded that students had more OLMs in the parts where they were more active and had more responsibility. Also, the order of the constituents of OLMs might be important as their size.