The mindfulness literature is now burgeoning and underlines various positive effects of mindfulness practice on mental health and well-being. Emotion regulation has been identified as a central capacity that develops through mindfulness practice, explaining the increase in positive emotions and the decrease in negative emotions. Furthermore, the results also show a decrease in the intensity of reactions and interference created by positive and negative stimuli, neutral evaluations, and an increase in emotional stability. It has been demonstrated, through neurological measures among other things that mindfulness leads to a specific type of emotion regulation, where the relationship between the individual and his emotions is profoundly and early modified. Equanimity was then identified as a possible explanation for the specificity of mindful emotion regulation. Since then, the literature on this subject has remained scarce, and existing experimental studies have not studied it. However, equanimity as a stable, calm mental state, unbiased by the valence of stimuli, seems to be an essential component of the experience of emotion in mindfulness. The objective of this thesis was to consider equanimity as a quality of emotion regulation related to mindfulness, to demonstrate its presence in the existing literature, and to offer the first bases for its study in experimental psychology. We first identified an operationalizable definition of equanimity and validated a questionnaire to measure its level in meditating and non-meditating individuals. We then examined the relationship between the practice of mindfulness and the level of equanimity. Third, we used an approach and avoidance task to link equanimity to a regulation of motivational tendencies toward positive and negative stimuli. Finally, in order to explore equanimity in relation to the regulation of health behaviours, we studied its impact on the evaluation of several types of foods. The results of our studies showed that equanimity increased with the practice of mindfulness, and that it was related to the decrease of the approach and avoidance bias towards positive and negative words. Moreover, equanimity was also expressed by more neutrality in the hedonic evaluation of words and in healthier evaluations of food. This thesis provides a portrait of equanimity, which will hopefully encourage numerous studies on this subject.