This study aimed to evaluate the effect of bromoform (CHBr₃) dose on in vitro rumen fermentation and on CHBr₃ and dibromomethane (CH₂Br₂) concentrations in solution and the gas cap. In vitro treatments consisted of CHBr₃ (DOSE: 0, 1, 10, 100, 1000, 10,000 µg of CHBr₃), with five replicates per DOSE at each time-point (TIME: 0, 0.25, 0.5, 1, 2, 3, 4, 5, 6, 12, 24, 48, and 72 h). The 10,000 µg CHBr₃ DOSE inhibited fermentation completely and was removed from the dataset. The acetate:propionate ratio, nitrogen, and methane (CH₄) produced per gram of DMD decreased as DOSE increased (p = 0.01). As the DOSE increased, CH₄ decreased, and H₂ increased in a dose-dependent manner (p < 0.01). The CHBr₃ concentration dropped below the detection limit within 3 h of incubation. Dibromomethane concentrations for DOSE 1000 and 100 µg of CHBr₃ increased in solution and gas cap beginning at 0.25 h and 1 h post incubation and plateaued by hour 3 and 5, respectively (p < 0.01). The addition of CHBr₃ alters the molar proportion of volatile fatty acids, decreases CH₄, and increases H₂ production, and it is dehalogenated to CH₂Br₂ within 3 h of incubation in an in vitro system.