The presented paper deals with development of a water soluble core material which can be used for aluminum castings. During the development a suitable binder and refractory material has been chosen, their best ratio and firing cycle settled and a suitable impregnation solution found. Triangle of three mutually interconnected parameters has been resolved finally: STRENGTH – SHRINKAGE – SOLUBILITY, to find its optimal balance. Application trials were run in several investment casting foundries with focus on wax removal (autoclave or MW oven), shell annealing, pouring, core removal and casting soundness. Metallurgical research was carried out afterwards to evaluate interaction between the core material and aluminum alloy and surface roughness of the casting.