Since the introduction of the experience sampling method (ESM), there have been concerns that the repeated assessments typically related to this method may alter the behavior, thoughts, or feelings of participants. Previous studies have offered mixed results with some studies reporting reactive changes while others failed to find such effects. Our aim was to investigate under which circumstances ESM induces reactive effects.Students (N = 151) were randomly assigned to receive a questionnaire containing 30 or 60 items 3, 6, or 9 times per day for 14 days. A random sample of 50 participants took part in qualitative interviews after the end of the data collection. We investigated changes over time in the data, while taking into account the sampling protocol and characteristics of participants, and analyzed qualitative reports of measurement reactivity.Decreases in completion time, within-person variance of ratings and subjective reports of habituation point towards the existence of a habituation period. While participants also reported increases in emotional awareness in interviews, ESM measures indicated a decrease in emotional awareness over time. Changes in behavior were rare in quantitative and qualitative reports. Positive affect was found to decrease over time in the ESM data and various changes in affect, emotion regulation, and thoughts were reported in interviews. Individual characteristics and sampling protocol had inconsistent effects on changes over time.The current results suggest that ESM induces reactivity. These reactive changes may be particularly relevant to researchers investigating within-person variability, completion times, affect, or emotional awareness.