The aim of the paper is to explore perceptions of slavery in zones of interaction between European and indigenous communities in the mid-colonial era, seen through the lens of missionary writings and focusing on the Timor Residency in the Dutch East Indies in the 19th century. While slave trade was formally abolished in the Dutch colonies shortly after the British interregnum, this largely became a dead letter as slaves were acquired and traded until at least the 1830s by Dutch interests, and until after 1900 by indigenous groups. This was in particular the case in the areas where colonial control was weak or non-existent. The colonial contact zones in eastern Indonesia, such as Kupang in Timor, housed considerable slave populations until the second half of the 19th century, and slave labour was an essential part of the household economy of the European or Eurasian burghers. The paper studies how the position and work of slaves were perceived by Reformed Christian missionaries of the Nederlandsch Zendelinggenootschap (NZG) who were active in the Timor area since 1819. A large body of letters and reports have been preserved at Utrecht and provide a comprehensive picture of how slaves interacted with their superiors in daily life. This material is written in a more independent and critical style than official colonial reports, and is often more initiated than travel accounts. The European missionaries, driven by ideals of personal piety, encountered the moral dilemma at their arrival, how to fit into a society where slavery was widely accepted as a fact of life. The paper shows how the views in the letters spans over a range of attitudes, from reluctant acceptance to utter disapproval, and discusses what this material may tell us about the actual conditions of unfree labourers in a hybrid milieu.