UX_Mate (UX Motion Activation Tracking Engine), is a software tool developed for automatic assessment of UX by means of facial motion tracking. UX_Mate brings together the advantages of EMG and approaches based on video analysis since it does not require invasive devices and can be used in natural settings, including situations with critical or varying illumination conditions. Moreover, it exploits fine-grained facial motion tracking instead of relying on a fixed emotion classifier. This feature allows taking advantage of low-intensity, mixed emotions as the ones elicited in HCI. A database of annotated and synchronized videos of interactive behavior and facial expressions, and details about the evaluation are available on our research paper.