We present the results of gas phase electric dipole measurements for a series Of M18X17 alkali-halide clusters. This cluster has a large structural dipole, which is due to the periodic arrangement of the ions in the nano-crystal 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. (C) 2002 Elsevier Science B.V. All rights reserved.