Hoolocks are the second largest of the gibbons, after the Siamang.

They reach a size of 60 to 90 cm and weigh 6 to 9 kg. The genders are about the same size, but they differ considerably in coloration: males are black colored with remarkable white brows, while females have a grey-brown fur, which is darker at the chest and neck.

White rings around the eyes and around the mouth give their face a mask-like appearance. Young hoolocks are born after a seven month gestation, with a milky white fur.

After about six months their fur turns black. After 8 to 9 years they are fully mature and their fur reaches its final coloration. Their life expectancy in the wild is about 25 years.