Formerly, the western grey gibbon (H. abbotti) and eastern grey gibbon (H. funereus) were considered conspecific with H. muelleri, but more recent studies indicate that all three are distinct species, and both the IUCN Red List and the American Society of Mammalogists consider them such.

Weighing between 4–8 kg, it ranks among the smaller of the gibbons.

[7] Müller's gibbon is endemic to Kalimantan in the island of Borneo, inhabiting the southeastern part of the island.

[2] Southern grey gibbons are diurnal rain forest dwellers, characterized by the long arms that all gibbons have, with which they brachiate through the trees.

They live together in monogamous pairs, and defend their family territory against intruders with long, loud singing.