Photosynthetically active radiation (Q)-use efficiency (epsilon) is an important parameter for deriving carbon fluxes between forest canopies and the atmosphere from meteorological ground and remote sensing data. A common approach is to assume gross primary production (P(g)) and net primary production (P(n)) are proportional to Q absorbed by vegetation (Q(abs)) by defining the proportionality constants epsilon(Pg) and epsilon(Pn) (for P(g) and P(n), respectively). Although remote sensing and climate monitoring provide Q(abs) and other meteorological data at the global scale, information on epsilon is particularly scarce in remote tropical areas. We used a 16-month continuous CO(2) flux and meteorological dataset from a mountainous tropical rain forest in central Sulawesi, Indonesia to derive values of epsilon(Pg) and to investigate the relationship between P(g) and Q(abs). Absorption was estimated with a 1D SVAT model from measured canopy structure and short wave radiation. The half-hourly P(g) data showed a saturation response to Q(abs). The amount of Q(abs) required to saturate P(g) was reduced when water vapor saturation deficit (D) was high. Light saturation of P(g) was still evident when shifting from half-hourly to daily and monthly time scales. Thus, for a majority of observations, P(g) was insensitive to changes in Q(abs). A large proportion of the observed seasonal variability in P(g) could not be attributed to changes in Q(abs) or D. Values of epsilon(Pg) varied little around the long-term mean of 0.0179 mol CO(2) (mol photon)(-1) or 0.99 g C MJ(-1) (the standard deviations were +/- 0.006 and +/- 0.0018 mol CO(2) (mol photon)(-1) for daily and monthly means, respectively). In both cases, epsilon(Pg) values were more sensitive to Q(abs) than to daytime D. These findings show that the current epsilon-approaches fail to predict P(g) at our tropical rain forest site for two reasons: (1) they neglect saturation of P(g) when Q(abs) is high; and (2) they do not include factors, other than Q(abs) and D, that determine seasonality and annual sums of P(g).