This work presents the first dual fuel measurements with pure jatropha oil and biogas, using a 12kW diesel engine generator. Reference tests are done with pure jatropha oil and with diesel to characterize the engine’s thermal efficiency η_t, volumetric efficiency η_v and air-excess ratio λ versus output power. An extensive parameter study is done to predict/explain the effect of dual fuel operation on η_v and λ. Dual fuel experiments, adding different qualities (CH₄/CO₂ ratios) of synthetic biogas to the intake air, show that thermal efficiency is hardly affected for higher loads. For lower loads, biogas addition results in a decrease up to 10% in thermal efficiency, independent of biogas quality. Both η_v and λ decrease with addition of biogas, in quantitative agreement with predictions. The engine runs well up to a certain heat release fraction of methane; at higher fractions irregularities are observed, probably attributable to light end-gas knock.