A thermal infrared thermometer was used to record the passage of hot gases and fragments across a measurement area located at the exit of one of Yasur's active vents. Recording was completed over 2 h during September 2011. A total of 200 explosive events were recorded. We define two types of event: low-energy events with typical thermal energies of 14 kJ and high-energy events with typical thermal energies of 97 kJ. Around 180 low-energy events were recorded, which together released 457 kJ of radiant energy. In contrast, only 20 high-energy events were recorded, but they released 2042 kJ. We suggest that low-energy events originate from a relatively shallow, degassed magma reservoir and are associated with bursting of bubbles formed by bubble coalescence during ascent. Instead, high-energy events originate from a relatively deep, fresh magma reservoir and are associated with slugs formed by foam collapse.