This paper is based on ongoing research on a recent low-dose, late partial English immersion (IM) program in Germany. The evaluation compares English language outcomes of IM groups, groups from non-IM schools, and non-IM groups from the same school as the IM groups, at various points of their development. This paper focuses on whether English vocabulary learning occurs incidentally while students are learning history or geography, or both, taught in English and whether there is evidence to suggest that the learning abilities activated in the IM classroom are the same as those found in traditional foreign language teaching and in naturalistic (untutored) L2 acquisition. The data derive from a communicative group test. It is shown that some of the lexical items cannot have come from the textbook or from other kinds of teaching materials used during regular foreign language instruction in the program. This leaves the teacher's oral use of English as the most likely source. Several implications for L2 acquisition theory and teaching practice are discussed.