The carbon-isotope composition of both forest and grassland soils from Papua New Guinea exhibit predictable trends with increasing altitude. Soils under pure C3 vegetation (forests and alpine grasslands above 4000 m) show an increase in &dgr;13C value with altitude paralleling the increase in &dgr;13C value observed in plant leaves by K¿rner et al. [1988]. Grassland soils show a decrease in &dgr;13C value above about 1000 m, from maximum values which are close to pure C4 values (-12 to -13%0 vs PDB) to minimum values which are indistinguishable from pure C3 values at 3500--4000 m (~-26%0). Within this general framework, several factors can influence the soil &dgr;13C value at individual locations. In local forest settings, soil &dgr;13C values will be influenced by the degree to which respired CO2 is re-utilized during photosynthesis, the proportions of leaf and wood litter, and the degree of decomposition. In grassland settings the primary factor controlling the observed &dgr;13C variability at any specific altitude is the amount of nongrass C3 carbon present in the sample. It is also possible that other factors, such as moisture availability, may play some role in determining the proportions of C3 and C4 grasses at any given altitude, although further work would be required to substantiate such a link. The results provide a framework within which to more accurately constrain the carbon-isotope composition of terrestrial carbon pools and to interpret variations in the isotopic composition of riverine particulate organic carbon.