This paper focuses on the morphological transparency of the German writing system and its relationship to learning processes in the areas of morpho- logical awareness and decoding skills, especially with regard to multilingual learners. We present results of an intervention study in grade 5, which was fo- cused on the analysis of morphological structures of complex written words. The results show that the present intervention, as compared to controls (who worked on complex words as suggested by current textbooks), fostered morphological awareness with respect to all three assessed sub-dimensions (i.e., forming word families, identifying root words, segmenting words into morphemes). After five months, only effects on the identification of root words showed again in the follow up. The findings also indicate a significant improvement in the achievement of the multilingual learners in the intervention group in segmen- ting words into morphemes. The results of the study suggest that future teach- ing concepts have to put more emphasis on morphological structures.