We investigated the variation of stable (δ¹³C) soil carbon isotopes in relation to depth in seven of the most important savanna areas to adjacent contiguous forests in the Amazon region. The δ¹³C of bulk organic matter in all profiles from forested sites increased with soil depth. In forest profiles from Amapá, Alter do Chão, and Roraima, the enrichment was less than 3.5‰ between deeper soil and surface layers, suggesting that C₃ plants have remained the dominant vegetation cover. On the other hand, in forest soil profiles from Humaitá and Carolina sites, the δ¹³C enrichment was greater than 3.5‰, indicating the influence of past C₄ vegetation or a mixture of C₃/C₄ vegetation (woody savanna). The surface δ¹³C values in the savanna profiles were 5–13‰ greater than the comparable forest profiles, indicating the influence of C₄ vegetation. Two kinds of isotopic distribution were observed in deeper layers. The savanna profiles at Alter do Chão, Chapada dos Parecis, and Redenção had relatively constant δ¹³C values throughout the profile, suggesting minor past changes in the vegetation composition. In profiles at Amapá, Roraima, Humaitá, and Carolina, δ¹³C values decreased with depth from the surface and converged with comparable forest values, suggesting more woody savanna in the past than exists currently.