In this study, we present a comparative corpus-based analysis of the English /s/ ~ /z/ voice contrast for learners of three L1s. Acoustic analysis of periodicity and duration for target segments confirms expectations based on L1 transfer and on the Markedness Differential Hypothesis. We found that, due to the absence of phonemic /z/ in Spanish, L1 Spanish learners exhibit great difficulty in producing voiced realisations for /z/, and more so in the (more marked) word-final position than in the (less marked) word-medial position. In contrast, L1 French and L1 Italian learners did not exhibit difficulties in reproducing the voicing patterns of English /s/ ~ /z/ neither word-medially nor word-finally, due to the existence of these sounds in their L1 (and despite differences in relative markedness for these two positions, especially considering that word-final /z/ does not exist in Italian). Finally, we observed the impact of orthography on the production of L1 French and L1 Italian learners, affecting the periodicity of /s/ and /z/ depending on spelling transparency.