This study examines the factors influencing language and script choice in instant messaging (IM), a form of real-time computer-mediated communication, in a multilingual setting. Grounded in the New Literacy Studies, the study understands IM as a social practice involving texts, encompassing a range of literacy practices, within which a subset called "text-making practices" is highlighted in this article. Drawing on results from an analysis of chat texts, interviews, and logbooks collected from 19 young people, the author suggests that the text-making practices related to language and writing system choice are guided by the perceived affordances of the IM technology and the available linguistic resources. Seven ecological factors influencing these perceptions have been identified: "perceived expressiveness of the language", "perceived functions of IM", "user familiarity with the language", "user identification with the language", "technical constraints of inputting methods", "speed", and "perceived practicality of the writing system". The author argues that these factors often co-occur in real use. (Contains 2 figures, 1 table, 7 excerpts, and 2 notes.)