The objective of this study was to determine whether supplementing organic acids can prevent incidences of subacute ruminal acidosis (SARA) in beef heifers fed a diet consisting of 8% barley silage and 92% barley grain-based concentrate (DM basis). Ten ruminally cannulated Hereford crossbred heifers (484 +/- 25 kg BW) were used in a replicated 5 x 5 Latin square design with 14-d periods including 10 d for dietary adaptation and 4 d for measurements. Dietary treatments included no supplementation (Control), low fumaric acid (61 g/d), high fumaric acid (125 g/d), low malic acid (59 g/d), and high malic acid (134 g/d). Organic acid supplementation had no effect on DMI (P = 0.77). Similarly, no effects were observed on mean (P = 0.74), minimum (P = 0.64), and maximum (P = 0.27) ruminal pH measured continuously for 48 h. Moreover, area under the curve for pH thresholds 6.2 (P = 0.97), 5.8 (P = 0.66), 5.5 (P = 0.55), and 5.2 (P = 0.93) was similar for all treatments. However, malic acid supplementation lowered the amount of time that ruminal pH was < 6.2 compared with the Control (P = 0.02) and fumaric acid treatments (P < 0.01). No effects were observed on total VFA concentrations with organic acid supplementation (P = 0.98) compared with the Control, but greater total VFA concentrations were observed with fumaric acid compared with the malic acid treatments (P = 0.02). The population of total culturable bacteria 3 h after feeding was reduced with supplemental malic acid compared with the Control (P = 0.03) and fumaric acid treatments (P = 0.03). However, no effects were observed with organic acid supplementation on lactic acid-utilizing bacteria (P = 0.59). In conclusion, under the conditions of the present study, organic acid supplementation did not have any significant effects on ruminal fermentation parameters compared with the Control and were not effective in preventing SARA in beef cattle fed high-grain diets.