The Aulnois rises in the forest of Muno which stretches over the south of the Belgian province of Luxembourg.

After crossing the Franco-Belgian border it continues towards the south until its confluence with the Chiers, which occurs at Carignan in the Ardennes just after the Aulnois has received its main tributary, the Matton.

The Aulnois flows through the French communes of Messincourt, Pure, Osnes and Carignan, all in the Ardennes département.

At its confluence with the Chiers, its mean annual flow rate or discharge rises to 2.10 cubic metres per second (74 cu ft/s) for a watershed of 101.8 square kilometres (39.3 sq mi).

[3] Its specific flow rate thus reaches the high value of 20.6 litres per second per square kilometre of watershed.