Research suggests that the decrease in milk fat synthesis observed in dairy cows fed rations high in concentrates or supplemented with vegetable oils could be prevented by increasing dietary cation-anion difference (DCAD) and/or potassium supply. The objective of this study was to evaluate the effect of potassium carbonate (K₂CO₃) on lactational performance of early-lactating dairy cows fed diet supplemented with soybean oil (SBO). Eight primiparous and 20 multiparous Holstein cows averaging 39 ± 9 DIM (Mean ± SD) were used in a randomized complete block design (7 blocks) based on DIM and number of calving with a 2 × 2 factorial arrangement of treatments. Within each block, cows were fed a basal diet formulated to achieve 40% forage (58% corn silage) and 60% concentrate (47% non-fibrous carbohydrates), with 0 (DCAD: +95 mEq/kg) or 1.5% K₂CO₃ (DM basis; DCAD: +316 mEq/kg), and 0 or 2% SBO. Effects of K₂CO₃, SBO and their interaction were evaluated. Treatment period was 28 d in length, plus 1 wk pretreatment collection period, used as covariate and the last 5 d used for data and sample collection. Dry matter intake was not affected by treatments (24.8 ± 1.2 kg/d; P = 0.56), but milk yield was increased when SBO was added to the diet (41.8. vs 38.6 kg/d; P = 0.01). Milk protein content was decreased by SBO (3.02% vs. 3.23%; P = 0.01), but a similar milk protein yield was observed among treatments (1.26 ± 0.01 kg/d; P = 0.19). Milk fat percentage (3.30 ± 0.07%; P = 0.21) was not affected by treatments. Feeding SBO tended to decrease milk fat yield, exclusively when cows were fed a diet without K₂CO₃ (0% K₂CO₃: 1.26 vs. 1.33 kg/d, 1.5% K₂CO₃: 1.31 vs. 1.34 kg/d; interaction: P = 0.09). 4% fat-corrected milk was increased with K₂CO₃ supplementation in cows fed SBO diets (37.4 vs. 35.0 kg/d), whereas the opposite effect was observed for cows receiving diets without SBO (35.4 vs. 36.9 kg/d; interaction: P = 0.06). Milk urea nitrogen was decreased by SBO (14.2 vs. 16.2 mg/dL; P = 0.03) and K₂CO₃ (13.9 vs. 16.5 mg/dL; P = 0.01). In conclusion, the effect of K₂CO₃ on milk production and composition is affected by dietary unsaturated fatty acid supplementation.