The Sibe of Xinjiang have been recognized as speakers of a Manchu variety by linguists. However, for the Sibe speakers themselves, the situation is more complicated. For certain reasons, the Sibe often present themselves as a group whose historical origins are different from the Manchus. Several mentions occur in historical sources about Sibe being vassals to the Khorchin Mongols before “becoming Manchus”. This has been used among the arguments for the non-Manchu identity of the Sibe. In recent years, academic discussion has focused on the ethnic identity of the Manchus, and, to a lesser extent, also on the position of the Sibe in relation to the Manchus. In this paper I try to select out features of possible Khorchin, i.e. eastern Mongolian, origin, in Sibe which may have come from direct language contact. I discuss several morphological features of Mongolic origin which seem not to be shared by other Manchu varieties, and one remarkable Sibe feature of Khorchin origin (the emphatic prefix me-). In addition, I mention the existence of lexical evidence of direct contact which is found in more conservative layers of Sibe vocabulary. Another question concerns the significance of this evidence for imagining the Sibe history. The linguistic situation in central Manchuria during the period concerned (15th–16th centuries) suggests that if the shared features indeed come from this period, they may rather be remnants of an extinct linguistic environment characterized by intense Mongolic-Tungusic contacts than of bilateral contact between two distinct groups – Khorchins and Sibe.