The influence of sodium on the growth path and the properties of Cu-poor prepared CuInS2 (CIS) thin films is investigated. In situ experiments of X-ray diffraction and Raman spectroscopy reveal a different growth path when sodium is incorporated. A delay of the CIS formation has been found as well as a different contribution of the resulting phases in the layer like the In-S alloy is now supposed to be located nearer the surface. Sodium doping results in a remarkable increase in the lateral conductivity and the cell efficiency of CuInS2 based solar cells. (C) 2003 Elsevier Science B.V. All rights reserved.