The present research examines the effects of a number of factors on crosslinguistic influence (CLI) in bilin' third language (L3) writing. The main focus is on the levels of adapted and non-adapted loan words as illustrative of CLI. A quantitative analysis explores the impact of typology and proficiency in the target and the source-languages. Language proficiency was evaluated in 399 compositions, 133 written in each language (Basque, Spanish and English). The adapted and non-adapted loan words found in English compositions were classified according to their source-language, word category, word class and type/token categories. The results pointed at language typology as the main predictor of the source-language of CLI. Additionally, a linear association was detected between the learners' degree of proficiency in the target language and the frequency of the CLI instances or adapted and non-adapted loan words. The overall results show that bilingual students use their entire linguistic repertoire to write in their third language (English).