We studied the effects of branched chain VFA supplementation, pH, and solids passage rate (K_p) on microbial activity in continuous culture using 8 fermenters in 4 periods. Treatments were: 1) control or BCVFA supplementation (2 mmol each/d), 2) high pH (ranging from 6.3 to 6.8) or low pH (5.7 to 6.2), and 3) either low (2.5%/h) or high (5.0%/h) solids K_pwere assigned in a 2 × 2 x 2 factorial arrangement of treatments. Liquid dilution was maintained at 10.0%/h. Fermenters were fed 50 g of a 50:50 concentrate:forage diet twice daily. Apparent OM degradation was not affected by any treatment. BCVFA treatment tended (P = 0.08, main effect) to increase NDF degradation, which also tended (P = 0.11) to be increased with higher pH but was not affected by K_p. Interaction among treatments for NDF degradation was P > 0.15. Apparent N degradation increased (P = 0.05) with high pH and decreased (P < 0.01) with high K_p. On low pH, BCVFA did not affect redox, but BCVFA increased (P = 0.02) redox when pH was high (interaction P = 0.09). Flows of bacterial N, nonammonia-nonbacterial N, and the efficiencies of bacterial N flow per kg of OM and NDF degraded were not affected by treatment (P > 0.15). BCVFA increased (P < 0.01) CH₄ emission rate from 1.5 to 10.5 h post feeding.Total VFA concentration in the fermenters tended (P = 0.06) to increase with high pH and decreased (P < 0.01) with High K_p. Total daily net production of VFA (subtracting doses) was not affected by treatment or treatment × feed. The net production of BCVFA was numerically lower when BCVFA was dosed, most likely due to use of the feed BC components directly. High pH increased (P < 0.05) isobutyrate and isovalerate production but decreased (P < 0.05) 2-methylbutyrate and valerate net production suggesting increased use by microbes. The anteiso FA precursor, 2-methylbutyrate, seems to be important for increasing membrane fluidity with increasing pH, and this BCVFA (unfortunately typically not separated from isovalerate in evaluation of nutrition experiments) appears to support a more critical role among the BCVFA to improve NDF digestibility with higher K_p and higher pH.