Studies of cognitive processes in SL writing have paid attention to the strategies that writers use to tackle problems in composing. In text-generation, finding lexical items in the L2 to express their meanings is one of the most crucial problems writers have to face, for reasons related to the availability and accessibility of relevant linguistic knowledge. However, L2 writers have a resource at their disposal: their mother tongue. Our study used think-aloud (TA) protocols to explore the strategic use of the L1 by a group of seven advanced Spanish learners of EFL engaged in solving lexical problems in two tasks: a narrative and an argumentative essay. We analysed the effect of task difficulty on the number and type (compensatory/upgrading) of lexical problems these writers tackled using their mother tongue and attempted to categorise the specific purposes for which the L1 was used. Our results indicate that the cognitive difficulty of the task plays a role in determining the number of lexical searches (LSs) undertaken and the type of search in which the writer uses the L1. The amount of L1 use is also affected by task. These findings will be interpreted from a number of perspectives including individual differences and theories of cognitive task complexity, multicompetence, and the language learning potential of L2 writing. (Contains 6 tables and 2 figures.)