This study examined how three different sources of stimulus variability—overall amplitude, fundamental frequency, and speaking rate—affect second language (L2) vocabulary learning. Native English speakers learned Spanish words in presentation formats with no variability, moderate variability, and high variability. Dependent measures were accuracy and latency of picture-to-Spanish and Spanish-to-English recall. The findings indicated that variability sources that do not affect first language (L1) word identification (amplitude, fundamental frequency) do not affect L2 vocabulary learning. Conversely, variability in speaking rate, which negatively affects L1 word identification, positively affected L2 vocabulary learning. These findings are consistent with an integrated account in which sources of variability that affect phonetically relevant properties of speech impede L1 speech processing but improve L2 vocabulary learning.