Just-in-time adaptive interventions (JITAIs) have the aim to provide individualized interventions at optimal moments in time. A complex systems approach suggests that moments of instability may be such an optimal moment in time. In this study, we tested whether a micro-JITAI was more effective when applied during affective instability, as identified in real time from ecological momentary assessment data. We analyzed data from 70 participants who reported their momentary affect 5 times per day during a 60-day period. Instability was identified in real time from the exponentially weighted moving standard deviation (EWMSD) of their momentary affect ratings. The pleasant activity selector was applied as micro-intervention during periods of instability as well as during semi-random control periods. The results showed that the micro-intervention was not effective in improving positive affect. Moreover, the EWMSD appeared very sensitive to negative outlying values of momentary affect, instead of solely to enlarged fluctuations (the hypothesized indicator of instability). We discuss the implications for future studies that aim to test the effectiveness of JITAIs during instability.