We report on investigations of the morphology of spin-coated thin films of an organic mixed ionic-electronic conductor consisting of the conjugated polymer methyl substituted ladder type poly(para- phenylene) (mLPPP) blended with a solid state electrolyte (Dicyclohexano18crown6 (DCH18C6), complexed with lithium trifluoromethanesulfonate (LiCF3SO3)). This blend system was successfully applied as active layer in light-emitting electrochemical cells (LECs). While thin films blends of the conjugated polymer and the pristine crown ether show a very smooth surface, the addition of LiCF3SO3 causes a pronounced surface roughening. Since such a distinct surface roughness can be the reason for a device failure mechanism that limits the device lifetime, this attitude is investigated by tapping mode atomic force microscopy (AFM). These studies are complemented with X-ray analysis by means of energy dispersive X-ray spectrometry (EDXS), in the scanning electron microscope (SEM), in order to get a better insight into the ion distribution within the blend layer and its influence on the surface roughness formed. (C) 2003 Elsevier Science B.V All rights reserved.