This paper attempts a periodization and dialectal attribution of Iranian loan words found in Tocharian A and B, two Indo-European languages attested in c. 10,000 fragments unearthed in Chinese Turkestan since 1892. More than 100 loan words are scritinized and classified in eight sections, according to their origin: Old Iranian (probably issued from the common ancestry of the ‘Sakan’ languages, Khotanese, Tumshuqese and Waxi), three different stages of Khotanese, ‘Śaka’, (the language of the Iranian invaders of northern India), Parthian, Bactrian and Sogdian. Tocharians had dealings with all neighbouring Iranian peoples, but Khotanese and its ancestors clearly exerted the most durable influence. No loan word from more remote dialects (e.g. Persian and Ossetic) can be evidenced. The predominance of war-related and political vocabulary among the loan words and the direction of borrowing, overwhelmingly from Iranian to Tocharian, both point to a political ascendancy of Sakan-speaking tribes, and later of Bactria, on Tocharians.