We explored the potential of Landsat Enhanced Thematic Mapper (ETM+) imagery to quantify the expansion of planted oil palm area and changes in aboveground biomass (AGB) in plantation and forest in Sabah, Malaysian Borneo, from 2000 to 2008. For comparison, a classification layer derived from an Advanced Land-Observing Satellite Phased Array type L-band Synthetic Aperture Radar (ALOS-PALSAR) Fine Beam Dual (FBD)-polarized mosaic from 2008 was used for change detection analysis. Field-measured AGB values from 85 ha of forest and oil palm plantation plots were compared with 12 vegetation indices (VIs) and four spectral mixture analysis (SMA) derivatives. Correlations against indices using optical data were higher for oil palm biomass than for forest biomass. Change detection analysis of forest conversion to oil palm plantation was performed for areas designated as protected areas, commercial forest reserve and areas with no forest-use designation. This analysis found an increase in oil palm area of 38% (1450 km2) and a total decrease in forest area of 13.1% (1900 km2) for the whole study area from 2000 to 2008. The greatest area of forest loss was in areas not designated as forest reserve by the Sabah Forestry Department, although some oil palm expansion was detected in both commercial and protected areas. Using derived equations for biomass, we estimated that 46.6 Tg of carbon dioxide equivalents (CO2e) were released in these three forest designations or 53.4 Tg CO2e for the entire study area due to forest conversion to oil palm. These results are presented as relevant for on-going efforts to remotely monitor the carbon emission implications of forest loss as part of the United Nations Framework Convention on Climate Change's (UNFCCC's) proposed mechanism, Reduced Emissions from Deforestation and Degradation (REDD).