This paper demonstrates how historical corpora can be used in researching language phenomena. We exemplify the advantages and disadvantages through exploring three of the available corpora that contain textual sources of Old and Middle Bulgarian language to shed light on some aspects of the development of two words of ambiguous class. We discuss their behaviour to outline certain conditions for diachronic change they have undergone. The three corpora are accessible online (and offline – for downloading search results, xml files, etc.).