This study examines the effect of explicit phonological instruction on the acquisition of variable /s/ voicing in Spanish by advanced Hungarian learners. Hungarian and Spanish have very similar, yet not identical, voicing assimilation (VA) systems; the most important difference lies in the pre-sonorant context as sonorant consonants trigger voicing assimilation in Spanish but not in Hungarian. Data were collected in acoustic experiments from 7 native speakers of Northern Peninsular Spanish and 12 Hungarian university students who were advanced learners of Spanish. The latter group was tested twice: before and after a three-month Spanish phonetics and phonology course. The data reveal that this amount of instruction is not enough for L2 speakers to overcome their first language VA system, which might be attributed, in part, to the variable allophonic nature of the process and the similarity between the two languages’ VA systems.