© 2019 Acoustical Society of America. This study examined the perceived similarity between Standard British English vowels and Spanish vowels, including Spanish diphthongs, not usually considered in studies on cross-language categorization. Twenty-nine Spanish speakers performed two perceptual assimilation tasks that differed in the number of response alternatives provided, that is, including or excluding diphthongs. The participants also performed an L1 vowel identification task. The results showed that Spanish listeners consistently perceived English diphthongs as closer to Spanish diphthongs than to Spanish monophthongs, with comparatively high assimilation scores. These results emphasize the need to include diphthongs in cross-language comparisons and second-language acquisition studies involving languages like Spanish.