This paper investigates how inner speech in English as a foreign language (LX) and Chinese first languages (L1s) of 425 multilingual Chinese university students in the UK is affected by their stay. An eight-item scale was developed to cover two different discourse domains for inner speech, namely the academic and the general domain. LX socialisation was operationalised as frequency of English use in daily life, sociocultural adaptation, previous immersion, and length of stay. Factor analysis of the quantitative data combined with participants' reports show that English inner speech develops gradually in the academic domain and general domain, suggesting that the Complementarity Principle applies to inner speech as much as articulated speech (Grosjean, "Bilingual: Life and reality." Harvard University Press, 2012). Frequency of academic English inner speech is linked to higher level of LX socialisation, namely frequent use of English in daily life, a higher level of sociocultural adaptation, and having had previous immersion. However, sociocultural adaptation had no effect on the frequency of general English inner speech.