This book presents the results of a field research on the verbal system of Soqotri, a little-studied language spoken on the island of Soqotra (Arabian Sea) and belonging to the Modern South Arabian branch of Semitic. The investigation focuses on the so-called T-stems (marked by the infix -t-), mostly employed as derivational means of detransitivisation. In this book you will find comprehensive descriptions of the synchronic morphology and semantics of the T-stems, as well as an inquiry into their diachronic background. Simultaneously, the study is a contribution to the general typology of detransitivising derivation in the languages of the world.