Linguistic complexity has been investigated and measured at various dimensions of language (syntax, morphology, lexicon), but has not, until very recently, been considered at the linguistic interfaces. This research is a response to a recent call to widen the scope of L2 complexity research to the lexis-grammar interface (Paquot, 2018; Housen et al., 2018), building on Paquot (2018, 2019) who looked at the phraseological dimension of language in English as a Foreign Language (EFL) learner writing and found that measures of phraseological sophistication are better suited to index proficiency than measures of syntactic and lexical complexity, particularly at the B2 to C2 levels of the Common European Framework of Reference (CEFR). As a partial replication of Paquot (2018, 2019), we aim to establish the cross-linguistic validity of measures of phraseological complexity by assessing their effectiveness as indices of L2 Dutch (writing) proficiency. The research questions that we aim to answer are: 1. How effective are measures of phraseological complexity in indexing L2 Dutch (writing) proficiency? 2. How do measures of phraseological complexity compare to traditional measures of syntactic and lexical complexity in L2 Dutch written productions? 3. How does the development of phraseological complexity in L2 Dutch compare to the results observed for L2 English in Paquot (2018, 2019)? This investigation expands on the metrics of lexical and syntactic complexity seen throughout L2 research by applying phraseological measures of diversity and sophistication outlined in Paquot (2018, 2019). The corpus used in this study is compiled from more than 2,700 written extracts (ca. 700,000 words) of the Dutch certification exam CNaVT (Certificaat Nederlands als Vreemde Taal), produced by learners of Dutch from multiple L1 backgrounds at the B1-C1 CEFR levels. Quantitative measures of phraseological diversity (root type token ratio—RTTR) and sophistication (MI-based) are computed for the target phraseological units, and are then ...