A recent hypothesis posits that temperature-dependent sex determination (TSD) in reptiles may be maintained if females can assess thermal conditions and lay eggs accordingly to produce the sex benefiting most from maternal investment. Specifically, females should lay large eggs in environments likely to produce the sex benefiting most from larger egg size. This relationship has been demonstrated in diamondback terrapins. To evaluate the generality of this hypothesis, we examined two components of this hypothesis using painted turtles (Chrysemys picta), a reptile with TSD. Vegetation cover around nests at oviposition significantly and negatively influenced nest temperatures during embryonic development, particularly during the period of sexual differentiation. However, no relationship existed between egg mass and either nest temperatures or vegetation cover around nests at oviposition. Therefore, large eggs were not laid in specific thermal microenvironments as a mechanism to match egg size with offspring sex ratio. These results do not support a general relationship between thermally based nest-site choice and maternal investment (egg mass) in reptiles with TSD.