Background: Previous research has shown that males slow more throughout the course of a marathon than females. Proposed reasons for differences in slowing include the fact that females oxidize proportionately more lipids and fewer carbohydrates during exercise when compared to males, and possible differences in thermoregulation. Respiratory exchange ratio (RER) can be used to estimate the ratio of fat to carbohydrates being metabolized. Purpose: To compare the degree of slowing (time in the first vs. second half of a marathon) between men and women, and determine if steady-state RER or ambient temperature differences predict the rate of slowing in male and female novice marathon runners. Methods: Chip times for 123 female and 44 male recreational marathon runners (21.0 ± 1.7yrs) were used to determine change in pace observed in the second half of the marathon compared to the first half. A two-mile time trial (2MI) was used to assess baseline fitness and pace for steady-state measurements. A submaximal 6-minute treadmill run at 75% of 2MI velocity was completed 1-3 weeks before the marathon. RER was collected using a metabolic cart (Medical Graphics Diagnostics, St. Paul, MN). Baseline measures and outcomes (RER and percent slowing) were analyzed using independent samples t-tests to detect differences between the groups (men vs. women and by year 2014, cool weather vs. 2015, warm weather). Univariate ANOVA tests were run to analyze the differences in percent slowing (%slowing) and RER by year and sex. Pearson’s Product Moment Correlation Coefficient (r) was used to determine the strength of the relationship between RER and %slowing as well as the relationship between %slowing and percent body fat (%BF), weight, height, body surface area (BSA), and BSA to mass ratio (BSA/M). Results: The mean %slowing for the total sample for 2014 and the total sample in 2015 was 14.1± 12.0% and 22.0 ± 16.5%, respectively (p<0.05). The mean %slowing for the combined group from 2014 and 2015 males and females was 20.6± 14.8% and 17.02 ± 14.8%, respectively (p <0.05). Females had a significantly lower RER during steady-state exercise in comparison with males (Female = 0.87 ± 0.05, Male = 0.89 ± 0.05, p<0.05). Sex and year were predictors of %slowing. There was no significant relationship between RER, temperature of marathon, weight, %BF, BSA, or BSA/M and %slowing in the total group, but RER and height were significantly related (p<0.05). Conclusion: Consistent with previous research, males slow more than females from the first to second half of the marathon. However, RER was not associated with slowing during the marathon. Temperatures of the race did affect the rate of slowing, but men and women were not affected differently. This suggests that pace maintenance is not due to substrate metabolism.