The aim of this project is to investigate vocal abilities of white-handed gibbons (Hylobates lar) in Khao Yai National Park, Northeast Thailand. White-handed gibbons produce duet songs on a daily basis, thought to function in pair-bonding and home range defence, and specific songs in response to predators. Previous fieldwork showed that neighbouring groups frequently respond to duet song by duetting themselves (Raemaekers & Raemaekers, 1985) and it was observed in the field that sometimes they respond to predator songs by singing their own predator song, without having seen a predator themselves. These two song types are both composed of the same units but arranged in a different way. Based on these observations, white-handed gibbons may discriminate between predator song and duet song and understand the meaning of neighbours’ songs. A playback protocol was designed to simulate a natural event: a neighbouring group’s duet song or predator song will be played, and then we will observe the behaviour of the target group in response to the stimulus in order to investigate vocalization coding and call inference in gibbons. Investigating how gibbons code their songs and what inference other gibbons can make about these vocalizations could help explain the emergence and the current complexity of human language.