For marking materials such as balloons, a solvent-free or low-solvent marker must be used. Since we cannot estimate exactly what material the balloon is made of, we cannot give a safe recommendation. In addition, we do not have a completely solvent-free pen in the edding range. Nevertheless, there are some pens that have a water-based and low-solvent ink. We could recommend the edding 30/33 Brilliant paper marker.