A simplified analysis and data acquired in the 4.5 s drop tower in MGLAB, Japan in a quiescent oxygen/ nitrogen environment are presented for the prediction of the flammability limit in a quiescent microgravity environment. In the experimental matrix the oxygen level and thickness of PMMA are treated as control parameters. Published data from quiescent microgravity experiments on thin ashless filter paper and thick PMMA are also compared with the prediction of the analysis. Based on scale analysis, it is hypothesized that all fuels-from PMMA to cellulose- behave as thermally thin fuels during steady spread of flames in a quiescent environment. An expression for the spread rate that includes radiative effects is proposed for the first time: 710 1/2 + 1/2 VI -4%0, where eta(0) is the spread rate non-dimensionalized by its thermal limit and R-0, is the non-dimensional radiation number. For 91, > 1/4, which in dimensional terms translates to a critical thickness criterion tau > (F-2/4)(p(g)c(g)/p(s)c(s)(lambdag/epsilonsigma)[(T-v - T-infinity)/(T-v(4) - T-infinity(4))], flame extinction occurs irrespective of all other environmental conditions. Based on this prediction, an extinction thickness can be calculated even at 100% oxygen level. The experimental data from the MGLAB agree reasonably well with this prediction. Flammability maps with fuel half-thickness and oxygen level as coordinates are developed for PMMA and cellulosic fuels, which are shown to be consistent with the current and published data. (C) 2003 The Combustion Institute. All rights reserved.