Studies to date indicate that palm oil plantations support a lower diversity of wildlife compared to other land uses. Programs for certified palm oil have been established in order to increase environmental sustainability of palm oil production, reduce adverse environmental effects, and encourage "wildlife friendly" palm oil plantations. Certification requires palm oil companies to set aside forest reserves based on High Conservation Value (HCV). A High Carbon Stock (HCS) approach also has been used voluntarily by some palm oil companies to prioritize their designated forest reserves. We use camera trapping to investigate mammalian occupancy and diversity in oil palm plantations and HCS and HCV forest reserves in West Kalimantan, Indonesia. We found significantly lower mammalian species richness in plantations relative to forest reserves, and the differences between mammal communities in palm oil and forest were much stronger in this study than in previous studies. None-the-less, within forest reserves, most mammalian species were found in low abundance. No correlation was found between mammal abundance and carbon stocks in forest reserves, but there was a significant correlation between mammal abundance and forest reserve size. Size of remnant forest fragments was the most important variable explaining the presence of large mammals. Our results suggest that designating forest reserves based on carbon stocks alone will not be sufficient to conserve mammalian biodiversity in palm oil landscapes. In contrast, the size of the forest patch should be considered when designating forest reserves. In my study area, most forest reserves currently are small and have poor long-term prospects for maintaining large mammals.