The effects of oils extracted from the trunk of Copaifera langsdorffii (Copaiba; 21.3% β-caryophyllene, a essential oil), and from the fruit of Pterodon emarginatus (Sucupira; 5.3% β-caryophyllene) on rumen fermentation were evaluated in 2 independent in vitro experiments at doses CONTROL, LOW, MEDIUM and HIGH (0, 30, 300, and 3000 mg/L of buffered rumen fluid, respectively). The oils were dissolved in 2.5 mL of ethanol, also added to CONTROL. The incubation was repeated 4 times in each experiment. The diet (50:50 concentrate:roughage ratio, 90% DM, 20% CP, 2.2% EE, 36% NDF) was ground (1 mm) and incubated using filter bags for 3, 6, 12, 24, 48, 72 and 96 h. Rumen fluid was collected from a cannulated Holstein steer fed the same concentrate:roughage ratio diet. Samples of culture fluid collected after 96 h were analyzed for N-NH₃ and VFA concentrations. The disappearance data of DM were used to calculate the degradability of DM (Orskov et al., 1980). The HIGH dose of C. langsdorffii oil, showed degradability of parameter b (potential degradability of the DM assumed to be degraded over time) 29% lower than CONTROL (63.4 vs 45.3%; P = 0.002) and the potential degradability (a + b, amount of DM which can be degraded within the rumen given sufficient time) was 15% lower than CONTROL (87.9 vs 75.2%; P = 0.001). No responses of C. langsdorffii oil was observed (P > 0.05) on effective degradability (at rates of passage of 5 and 8%/h) and on constituents of culture fluid (N-NH₃, total VFA, acetate, propionate, isobutirate, butirate, isovalerate, valerate and acetate:propionate ratio). In the P. emarginatus oil study, the response on parameter b was 45% lower for HIGH dose compared with CONTROL dose (60.4 vs 32.1%; P < 0.001). The potential degradability was affected by P. emarginatus doses (P = 0.013), it was similar to CONTROL and LOW doses (highest values, 83.0 and 83.4%, respectively), intermediate to MEDIUM (73.3%) and lower to HIGH dose (64.8%; 22% lower than CONTROL dose, P = 0.023). There were no effects (P > 0.05) of P. emarginatus oil on effective degradability and constituents of culture fluid. In conclusion, these phytogenic oils were able to modify microbial fermentation in the rumen environment.