Four experiments were conducted to examine the role of orthotactic probability, i.e. the sequential letter probability, in the early stages of vocabulary acquisition by adult native speakers and advanced learners of German. The results show different effects for orthographic probability in incidental and intentional vocabulary acquisition: Whereas low orthographic probability contributed positively to incidental acquisition of novel word meanings in first language (L1), high orthographic probability affected positively the second language (L2) intentional learning. The results are discussed in the context of the following concepts: (1) triggering the establishment of a new representation, (2) noticing of new lexemes during reading, and (3) vocabulary size of the L1 and L2 mental lexicons.