Purity is a recurring concept in Syro-Christian sources, to the point that its meaning and implications are too often taken for granted in scholarship. However, the understanding of purity, the social scope of its norms and the regulation of the relationship between pure and impure vary in each religious and social context. Syriac sources themselves offer a very varied picture, and suggest that authors utilized purity ideas in different ways based on their goals. This paper reflects on some of the challenges of analysing purity ideas in Syriac sources from the 4th to the 9th centuries. First, is it possible to provide a definition of purity valid diachronically for both East- and West-Syriac sources? And was there a specialized purity lexicon for the different kinds of purity (external, moral, ascetical)? It also presents the challenges of disentangling impurity from other concepts, such as sin. For instance, Syriac sources often preach the non-associations with heretics, considered a source of pollution. But is such a pollution only moral, or does it imply physical contagion as well? Even contact with heretical objects can in fact be polluting. Finally, the paper underscores the importance of separating modern-day concepts of purity from Syriac, ancient ones.