We present the results of gas phase electric dipole measurements for a series of M 1 8 X 1 7 alkali-halide clusters. This cluster has a large structural dipole, which is due to the periodic arrangement of the ions in the nanocrystal and is directly related to the size of the particle. Experimental values are compared to the prediction of a Born Mayer model and to ab initio calculations. The results for the various salts are reproduced by our calculations.