This study, which is part of a large-scale study of using objective measures to validate assessment rating scales and assessment tasks in a high-profile school-based assessment initiative in Hong Kong, examined how grammatical complexity measures relate to task type and analytic evaluations of students' speaking proficiency in a classroom-based assessment context. An in-depth analysis of oral performance on two different assessment tasks (i.e., monologic vs. interactive) from 30 English as a Second Language, Cantonese-mother-tongue, secondary school students was conducted using a range of measures of grammatical complexity derived from the previous second language (L2) speaking and writing studies. Results showed that the individual presentation task tended to promote not only a greater number of T-units, clauses, verb phrases, and words but also longer T-units and utterances, thus probably stretching learners more in terms of complexity of grammatical and lexical processing. Results also showed that complexity measures recommended as among the most useful complexity measures demonstrated no significant correlations with analytic ratings of learner speaking proficiency. These findings were then discussed in light of the complex, dynamic, and developmental nature of grammatical complexity as well as in light of a learner-, task-, and L2 form-sensitive account of L2 oral production. (Contains 4 footnotes, 5 tables, and 1 figure.)