The carbonates in the Middle Ordovician Ma5 5 submember of the Majiagou Formation in the northern Ordos Basin are partially to completely dolomitized. Two types of replacive dolomite are distinguished: (1) type 1 dolomite, which is primarily characterized by microcrystalline (\30 lm), euhedral to subhedral dolomite crystals, and is generally laminated and associated with gypsumbearing microcrystalline dolomite, and (2) type 2 dolomite, which is composed primarily of finely crystalline (30–100 lm), regular crystal plane, euhedral to subhedral dolomite. The type 2 dolomite crystals are truncated by stylolites, indicating that the type 2 dolomite most likely predated or developed simultaneously with the formation of the stylolites. Stratigraphic, petrographic, and geochemical data indicate that the type 1 dolomite formed from near-surface, low-temperature, and slightly evaporated seawater and that the dolomitizing fluids may have been driven by density differences and elevation-related hydraulic head. The absence of massive depositional evaporites in the dolomitized intervals indicates that dolomitization was driven by the reflux of slightly evaporated seawater. The d18O values (-7.5 to -6.1 %) of type 1 dolomite are slightly lower than those of seawaterderived dolomite, suggesting that the dolomite may be related to the recrystallization of dolomite at higher temperatures during burial. The type 2 dolomite has lower d18O values (-8.5 to -6.7 %) and Sr2? concentration and slightly higher Na?, Fe2?, and Mn2? concentrations and 87Sr/86Sr ratios (0.709188–0.709485) than type 1 dolomite, suggesting that the type 2 dolomite precipitated from modified seawater and dolomitic fluids in pore water and that it developed at slightly higher temperatures as a result of shallow burial.