Crews were dispatched at 3:05 p.m. to the apartment building located on the 1400 block of Delaware Avenue.
The Wilmington Fire Department hazmat technicians were then called to the scene and found unidentified, unlabeled chemicals in the basement.
A Department of Natural Resources representative was called to the scene and it was determined that a leak in an old refrigerant system was the source of the odor.
Crews stopped the leak and are ventilating the released refrigerant gas from the building.
Authorities say once the building is cleared of the leaked gas, a determination will be made on how the remaining refrigerant in the old system will be safely removed.
The situation was placed under control at 5:45 p.m., but the 1400 block of Delaware Avenue remains closed.
There are no reports of injuries.