In linguistically diverse contexts, individual language repertoires may embrace various levels of literacy skills in multiple languages and modes of representation, encapsulated as multiliteracies (New London Group, 1996; Cope & Kalantzis, 2000; 2009). This complex view of literacy urged the development of new theoretical perspectives on multilingual writing as being a synthesized competence, including all languages in a person’s repertoire, as dynamic and continuously evolving (Canagarajah, 2006; 2013; 2015). As of now however, no research reveals the empirical construct of multiliteracies. In our study, we aim to address this desideratum by investigating the relevant components of the construct of multilingual writing skills. We draw on data deriving from the German panel study “Multilingual Development: A Longitudinal Perspective (MEZ)” (Gogolin et al., 2017) on writing skills of multilingual students in three different languages: the majority language (German), heritage languages of migrants in Germany (Russian or Turkish) and the first foreign language all children learn at school (English). The MEZ study is a longitudinal cohort-sequence study with two starting cohorts (7th- resp. 9th-grade students) and with four waves of data collection. We will present the results of a structural equation modeling by which we investigated multilingual writing skills of 800 8th- and 10th-grade German-Russian and German-Turkish bilinguals from the second wave of data collection. The model includes the following indicators of writing skills in the three mentioned languages of the individuals: German, heritage language, first foreign language: task accomplishment, verbs, conjunctions, and number of written words. The results will transparently show the strength of our competence model of multilingual writing skills.