The effect of increased sulfate concentration in the imbibing water during oil recovery by spontaneous' imbibitions in different outcrop chalks at various wettability conditions at 130 degrees C has been determined. Core plugs from three chalk outcrops, Rordal, Niobrara, and Stevns, were aged in crude oil and included in this study. Stevns chalk exhibited increased oil recovery during spontaneous imbibition with increased concentration of sulfate in the imbibing water phase. The effect was less than reported by others and was wettability dependent. Spontaneous imbibition tests showed that the added oil recovery was greatest at Amott water indices below 0.2, and tests at and above 0.25 showed only minor effects from sulfate. Niobrara and Rordal chalk did not show increased oil recovery with increased sulfate concentration in the brine. These core plugs reflected more water-wet imbibition characteristics at elevated temperature, and the effect of sulfate could not be isolated. Measurements of Amott water indices before and after spontaneous imbibition at 130 degrees C exhibited increased water-wetness for Niobrara chalk at this elevated temperature. The wetting Preference for Stevns and Rordal chalk did not change after spontaneous imbibition at elevated temperature and maintained less water-wet spontaneous imbibition characteristics at ambient temperature, regardless if sulfate was present or not. The Rordal core plugs exhibited increased oil recovery by imbibition at elevated temperature, but the measured Amott wettability preference was stable. The results demonstrate that the effect from sulfate on spontaneous imbibition in chalk is dependent on the chalk type (i.e., rock mineral composition) and the wettability of the rock.