Numerous factors are thought to be advantageous for non-native language learning although they are typically investigated in isolation, and the interaction between them is not understood. Firstly, bilinguals are claimed to acquire a third language easier than monolinguals acquire a second. Secondly, closely related languages may be easier to learn. Thirdly, certain phonetic features could be universally more difficult to acquire. We tested these hypotheses used as explanations by having adults learn vocabularies that differentiated words using foreign phonetic contrasts. In Experiment 1, Mandarin–English bilinguals outlearned English monolinguals, and the Mandarin-like (retroflex) artificial language was better learned than the English-like (fricative voicing). In Experiment 2, bilinguals again outlearned English monolinguals for the Mandarin-like artificial language. However, only Korean–English bilinguals showed an advantage for the more difficult Korean-like (lenition) language. Bilinguals, relative to monolinguals, show a general advantage when learning ‘easy’ contrasts, but phonetic similarity to the native language is useful for learning universally ‘difficult’ contrasts.