There has been a great deal of academic discussion on the definition of the linguistic complexity of learner language. My paper uses the Finnish passive infinitive structure of tutkittaessa 'while doing research’ to represent the complexity in writing. I will examine the frequency of the structure and the accuracy of its use in the academic writing of advanced Finnish learners, and I will compare them to equivalent samples given by first language speakers of Finnish. I will also present a few structures which the learner may use incorrectly in replacing the structure in question. I will advocate the standpoint under which diverse morphosyntactic-semantic phenomena can be quite effective indicators of complexity.