In previous studies, the addition of potassium carbonate to continuous cultures of ruminal microorganisms decreased the production of trans-10 18:1 and trans-10,cis-12 conjugated linoleic acid (CLA), consistent with in vivo reports of higher milk fat percentages in lactating cows fed supplemental potassium carbonate. This experiment compared sodium carbonate to potassium carbonate to determine if they had similar effects on biohydrogenation intermediates. A single basal diet was fed (60 g/d) to 4 dual-flow continuous fermenters for 4 10 d periods divided into 7 d for adaptation and 3 d for collection of samples. The diet consisted of alfalfa pellets and concentrate mix (47/53, DM basis), and contained 3.3% (DM basis) soybean oil. At each feeding (0800 and 1600 h), 16 mL were injected into each fermenter containing either only distilled water (CON), 1.4 g carbonate as K₂CO₃, 1.4 g carbonate as Na₂CO₃, or 1.4 g carbonate as a 50:50 mixture of K₂CO₃ and Na₂CO₃ (MIX). The experiment was analyzed as a randomized block design with differences among treatments determined by 3 orthogonal contrasts: (1) control vs. carbonate; (2) MIX vs. both single carbonates; and (3) K₂CO₃ vs. Na₂CO₃. Carbonate sources did not differ (P > 0.05) so MIX, K₂CO₃, and Na₂CO₃ means were averaged to give a single combined carbonate value. CON and carbonate values differed (P < 0.05) respectively for culture pH (6.05 and 6.36), acetate (58.9 vs. 65.5 mol/100 mol), propionate (27.2 vs. 22.3 mol/100 mol), acetate/propionate (2.12 and 3.19), and production of trans-10 C18:1 (504.1 and 251.7 mg/d), trans-11 C18:1 (253.6 and 355.5 mg/d), and cis-9, trans-11 CLA (8.4 and 11.7 mg/d). CON and carbonate also differed (P < 0.10) in production of trans-10, cis-12 CLA (19.7 and 12.8 mg/d). The results showed that sodium carbonate was equally effective as potassium carbonate in reducing the production of biohydrogenation intermediates that have been linked to the cause of milk fat depression.