The purpose of this study is to investigate L2 learners' knowledge of complex word part analysis ("word-building"), with particular reference to two issues: suffix acquisition and to the use of word families as a counting tool. Subjects were two groups of EAP students in a Thai university. Results suggest that (1) the use of word families as a counting tool leads to highly misleading conclusions, especially with less proficient learners; (2) affix learning proceeds on the basis of stem to derived form and not vice versa; (3) the acquisition of stem + suffix occurs with certain particular suffixes before others; (4) such word-building schemas may depend on frequency of exposure. Implications for teaching are discussed. (Contains 6 tables.)