The effect of using multispectral waterfog (MWF) as a countermeasure against thermally guided projectiles has been studied using the simulation package called OPTSIM. Different parameter settings have been used to test the outcome of the duel between thermally guided projectiles and armoured vehicles using MWF. Simulations show that MWF can be an effective countermeasure against thermally guided projectiles. However, the, probability for the projectile to hit the target depends on how well the vehicle is covered by MWF and the surroundings.