When turtle eggs are incubated on media at different water contents, both the water potential and thermal conductivity of the medium change. Snapping turtle eggs were incubated fully buried in sand at constant water content (θ, 4% by mass) but different water potentials (ψ) to assess the effect of ψ on egg water exchange independent of medium thermal conductivity (k). Treatment ψs were fixed using an isopiestic technique. Over the course of incubation, change in fractional egg mass was positively correlated with Ψ. Total egg water exchange ranged from 0.0 to +0.5 g × g−1 of initial egg mass for viable eggs. Hatching success averaged 0.94 for the four highest Ψs but decreased for the two lowest Ψs. Hatchling and yolk mass were independent of water exchange above egg mass gains of +0.1 g × g−1 (Ψs ≥ −542 kPa) but decreased for egg mass gains less than +0.1 g × g−1 (Ψs < −542 kPa). Hatchling and yolk water content and incubation period were independent of egg water exchange for all treatments. The results are consistent with the hypothesis that the thermal conductivity of the incubation medium influences the water exchange of eggs.