Total productivity of oil palm growing at two sites in West Malaysia was assessed on a monthly basis over three to four years aided by a method for calculating monthly bunch dry matter production. The level and seasonal variations in production were compared with those predicted by a simulation model of oil palm growth which used solar radiation and leaf area index as the main inputs. Deviations between measured and modelled productivity were catered for by invoking changes in assimilate storage. The contribution to the seasonal yield pattern, by changes in assimilation capacity (via changes in radiation and leaf area index) and storage pool size were assessed for each site. The influence of variations in sink strength on the system is discussed.