A Lagrangian analysis approach is used to track changes in reactant mass fraction and temperature within ﬂuid parcels during premixed combustion. Time series of the reactant mass fraction and temperature along ﬂuid parcel trajectories, termed “thermochemical trajectories,” are calculated using data from direct numerical simulations of unconﬁned premixed ﬂames for two different turbulence intensities. Both of the turbulence intensities are high, with Karlovtiz and Damköhler numbers corresponding a broadened ﬂame. By analyzing time series of reactant mass fraction and temperature along ﬂuid parcel trajectories, we show that fuel consumption and temperature rise within ﬂuid parcels are frequently non-monotonic, resulting in frequent cooling events even after the temperature within a parcel has risen to 1400 K or more. Through an analysis of dynamical effects in the governing equation for the reactant mass fraction, this non-monotonicity is shown to be due to strong molecular diffusive processes resulting from the creation of large thermochemical gradients by turbulent advection. Implications of these results for the understanding of turbulent premixed ﬂame dynamics and structure are outlined.