Conduction heat transfer into a vertical free burning slab was measured as a function of time and distance from the leading edge, and its contribution to the known values of effective heat of gasification was analyzed. The present set of experiments was performed using clear and black PMMA (polymethylmethacrylate) samples. The heat conduction into the pyrolyzing surface was deduced from temperature measurements in the interior of burning slabs. The surface conduction heat flux decreased from 14 kW/m2for clear PMMA and 13kW/m for black PMMA att= 300s to almost 8kW/m2at around l = 1000s and then reached a plateau, indicating a substantial degree of unsteadiness in the early part of combustion of slabs. It is concluded that, when estimating burning rates in numerical calculations one has to be careful in selecting a value for the effective heat of gasification because of the generally unsteady nature of heat conduction into the solid interior.