The calculated distance (air line) between Montpellier and Aix-en-Provence is approximately 79 miles respectively 127 kilometers.
By car or train, the actual journey to Aix-en-Provence is certainly longer, as only the direct route (as the crow flies) between Montpellier and Aix-en-Provence has been calculated here.
Air line (approximately)
127 kilometers
69 nautical miles