The purpose of the study was to examine the relationship between VO(2) and RPE at the lactate threshold (LT) and maximal fat oxidation rate (FAT(MAX)) in untrained adults and determine the stability of the relationship across sex, age, and fitness status. A total of 148 untrained adults (mean age [year] = 30.5 ± 13.9, height [m] = 1.72 ± 0.08 m, body mass [kg] = 82.6 ± 20.5, body fat [%] = 28.7 ± 12.0) completed a continuous incremental VO(2) peak/LT protocol. Fat oxidation rates were determined using indirect calorimetry. The highest recorded fat oxidation rate was chosen as FAT(MAX). The breakpoint in the VO(2)-blood lactate relationship was chosen as LT. RPE was based on the Borg 6-20 scale. Bland-Altman plot analysis demonstrated that VO(2) FAT(MAX) systematically preceded VO(2) LT (mean bias = 1.3 ml kg(-1) min(-1)) with wide limits of agreement (+9.6 to -6.9 ml kg(-1) min(-1)). Multivariate ANOVA revealed a significant difference between VO(2) FAT(MAX) (12.7 ± 7.5 ml kg(-1) min(-1)) and VO(2) LT (14.1 ± 5.9 ml kg(-1) min(-1)) in the total sample (p = 0.04). There were no differences between the intensities when the sample was divided into sex, age, and fitness comparison groups (p values >0.05). RPE FAT(MAX) (9.4 ± 2.5) preceded RPE LT (10.4 ± 2.0) in the total sample (p = 0.008), but was not different across comparison groups (p > 0.05). The present data indicate that the highest rate of fat oxidation slightly precedes the LT in untrained adults. For exercise prescription, a Borg-RPE of 9-12 identifies both FAT(MAX) and LT.