Children who grow up learning a heritage language at home, and a majority language from school and peers, represent a case of bilingualism in which there is less distinction between a “first” and “second” language. This thesis investigates the similar contrasts within two separate heritage language communities, in order to observe patterns that occur between heritage language groups. A contrast pairing for each pair of languages was chosen: one for English-Polish heritage bilingual children and one for English-Chinese children. It was hypothesized that there would be cross-linguistic influence present, and that the acoustic cues of the English contrast would influence the production and perception of the heritage language contrast. Children between ages 7;0 and 9;6 performed a picture-naming task and a perceptual categorisation task, involving a phonetic continuum across a lexical minimal pair. The data include heritage bilinguals as well as control samples of monolinguals of each language. Results suggest that the heritage bilinguals reliably produced and perceived each contrasts. A degree of cross-linguistic influence was also present, and was seen both spectrally and temporally. In production, a comparison of vowel duration between heritage speakers and monolingual English speakers showed a significant effect (p < 0.001), with heritage speakers relying less on the temporal cue. This was not repeated in perception. The effect is evaluated with respect to phonetic assimilation, presenting evidence for a shared phonetic space in which mutual influence occurs.