Mechanisms governing the gasification to combustion regime transition in counter-current packed bed biomass systems are investigated in the current work. Single-particle experiments are performed with different oxidizer streams (air, mixtures of O2/CO2) to bring out the role of the extinction of the envelope non-premixed “volatiles” flame and simultaneous volatile-char combustion at the stagnation point on the transition to char oxidation in packed beds. Results indicate that for O2 fractions lower than 23% (w/w), the transition is governed by the extinction of the envelope diffusion flame. At higher O2 fractions, typically >32% (w/w), simultaneous volatile-char oxidation at the stagnation point drives the transition to char combustion. The strain rates computed at “gasification to char combustion transition” from earlier studies with air and O2 fractions ≤23% (w/w) in the oxidizer streams coincide with the strain rates obtained at the transition from classical envelope flame to a wake flame with single particles. Velocities at which simultaneous oxidation of volatiles and char was observed at the stagnation point with single particles subjected to oxidizer streams with O2 ≥ 32% (w/w) matches with the velocities at transition in the packed beds from the earlier studies. Simultaneous volatile-char oxidation and decrease in reaction zone thickness from a few particles to one particle diameter observed post-transition in packed beds support the theory. © 2020 Taylor & Francis Group, LLC.