This study presents a quantitative analysis of pre-emptive strategies used by first language speakers in talk addressed to second language speakers with moderate proficiency. The strategies identified in the conversations are reformulation, simplification, left-dislocation, and decomposition. The data come from simulated emergency calls in which an (L1) 'operator' instructed a (L2) 'caller' how to put a 'patient' (a manikin) in the recovery position. The conversations were coded for how many (and which) strategies the operators used and for whether or not the instruction was understood by the caller. The analysis showed that there is a relationship between the average number of strategies used and the chance of successful understanding; a greater number of strategies employed led to fewer understanding problems. The study shows that in this context of instruction, L1 speakers orient to potential problems of understanding for L2 addressees by using recognizable pre-emptive strategies, and that these strategies are effective in reducing the likelihood of understanding problems occurring.