This article introduces a new research project focusing on the Aramaic language of the Zohar—the most influential work in the literary canon of the Jewish mystical tradition. The Zohar is generally assumed by scholars to have been written in Spain during the final decades of the thirteenth century, although traditional kabbalists and Orthodox Jews believe that it was authored by its main protagonist, Rabbi Shimon bar Yohai, in second-century Palestine. While not disputing the late medieval provenance of the work, this article challenges the standard scholarly opinion whereby the Aramaic in which it was written was an artificially manufactured idiom, drawing its vocabulary, grammar and syntax indiscriminately from a variety of 'genuine' Aramaic sources. Part I of this study demonstrates that some of the most distinctive 'flaws' of zoharic Aramaic are attested in a wide range of much earlier Aramaic sources. Part II, which will be published separately, as a sequel, concerns the related question of the Palestinian setting of the Zohar.