The effective thermal conductivity (lambda(e)) and effective thermal diffusivity (chi(e)) of oil-palm-fiber-reinforced treated composites were measured simultaneously with the transient plane source technique from 50 to 110degreesC. The fibers of the composites were treated with sodium hydroxide alkali, silanol, and acetic acid. The experimental results for the different treated composites showed that there were variations in lambda(e) and chi(e) over this temperature range. However, the maximum values of lambda(e), and chi(e) were observed at 90degreesC, in the vicinity of the glass-transition temperatures of these composites. An effort was also made to predict the temperature dependence of lambda(e) and chi(e) through the development of an empirical model. The theoretically predicted values of lambda(e) and chi(e) for these composites were in excellent agreement with the experimental results over the entire range of investigated temperatures. Sudden increases in lambda(e) and chi(e) in the glass-transition region of these composites were indicative of the fact that the crosslinking density decreased and was at a minimum at the temperature at which lambda(e) and chi(e) showed their maxima. (C) 2003 Wiley Periodicals, lnc.