Thermal degradation of solid fuels is an issue of major importance in the prediction of fire ignition and growth. The pyrolysis rate is generally deduced from the energy balance at the surface of the solid material, while all or part of the in-depth loss into the solid material or the net radiation at its surface is disregarded. The aim of the present study is to improve the accuracy and predictive capability of solid fuel pyrolysis models. Both the steady and transient burning of thick clear polymethyl methacrylate (PMMA) slabs are investigated. First, experiments are conducted to evaluate quantitatively the energy flux components at the surfaces of steady-burning vertically oriented slabs. The total heat flux from the flame is measured, showing a decrease from 30.9 to 23.4 kW/m2 as the sample height increases from 2.5 to 20 cm. A specific procedure for estimating the surface reradiation of burning slabs is conducted, changing from the vertical to the ceiling configuration, where flame self-extinction occurs. At midheight of the slab, the surface reradiation heat flux is as high as 11.5 kW/m2, which corresponds to the emissive power of an equivalent blackbody at a temperature of 671 K, in accordance with current spectroscopic measurements. Local steady-burning mass loss rates are then deduced from the energy balance at the fuel surface and compared with direct measurements, showing good agreement. Second, the transient burning of thermally thick slabs of clear \PMMA\ is studied experimentally and numerically. Pure pyrolysis, for incident heat fluxes of 14 and 18 kW/m2, and flaming experiments are conducted. A one-dimensional numerical model is proposed to solve the problem of conjugated heat transfer into the semitransparent material, assuming that the exposed surface is a perfect emitter and a perfect absorber (soot-covered). The Schuster–Schwarzschild approximation is employed to calculate in-depth radiation, using data for the absorption coefficient of clear \PMMA\ with a very fine spectral resolution. Model results exhibit the same trend as that revealed in experiments for the rise in temperature in the sample and the regression rates. Results show that the surface temperature tends asymptotically to a constant value, which increases with the incident heat flux, and that steady burning occurs when the surface temperature saturates. Similar steady-burning regression rates are obtained for the 18 kW/m2 and flaming configurations. It is found that for the flaming case, the increase in the steady-burning regression rate due to higher incident heat flux and surface temperature is nearly compensated for by the decrease of regression rate caused by higher losses (outward reradiation and heat of gasification).