Spanish (L2) speakers have difficulty in perceiving the French vowel height contrast /e/-/Ԑ/. L2 perception models attribute this perceptual difficulty to native (L1) phonology. For example, the Spanish /e/ vowel, which is perceptually similar to both French vowels, is assumed to influence their perception through an assimilation process. This pattern of assimilation depends upon the degree of the similarity between the vowels. This research aims to shed light on the relevant aspects of L1 phonology and its relation to L2 phonology that can be operationalized to predict L2 perception. Traditionally, measures of perceptual similarity have involved comparing the acoustic or gestural properties of productions by groups of ‘referenced' native speakers of the two languages of interest. Here, we rather compute acoustic distances (based on F1 and F2 values) between the native productions by Spanish adolescents learning French (N=14) and a group of native French speakers. In addition to this distance measure, we also introduced a novel measure, the within-speaker variability or compactness in productions of a L1 vowel to predict L2 perception. The results revealed that individual productions of Spanish /e/ and their variability (or compactness) predicted L2 perception accuracy of the French /e/-/Ԑ/ vowels. Index Terms: L1 transfer, L2 perception, interphonology