Cover crops have long been grown for soil erosion and weed control under oil palm plantations. However, field information on the cover crop effect on soil carbon (C) emissions is lacking for oil palm plantations established on peat soil. Therefore, we investigated soil carbon dioxide (CO₂) emissions under different cover crops in an immature and mature smallholder oil palm plantation on peat in Riau, Indonesia. Soil CO₂ efflux was measured three times a year, using a closed chamber system under different cover crops, between oil palm trees. The mean soil CO₂ emissions in immature plantations treated with Amorphophallus muelleri (IM1), Crotalaria juncea (IM2), Calopogonium mucunoides (IM3), regularly cleaned (IM4) and Nephrolepis biserrata (indigenous plant, IM5) were 4.83 ± 1.76, 4.59 ± 1.44, 3.13 ± 0.17, 4.07 ± 2.20 and 6.24 ± 3.57 μmol m^-2 s^-1, respectively. Meanwhile, in the mature plantations, soil CO₂ emissions were 2.96 ± 1.18, 3.66 ± 0.49, 4.36 ± 1.99, 2.20 ± 0.69, 4.05 ± 0.76 μmol m^-2 s^-1, respectively, similar to the above treatments, except for (M4), where frond stack was utilised.