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This study was undertaken as an effort to improve our understanding of the mechanisms of pulsating flame spread over liquids. A holographic interferometry technique was applied to obtain a time series of simultaneous temperature profiles in gas and liquid phases for pulsating spread over propanol. Liquid motion at the free liquid surface was visualized using a particle-track technique and recorded with a high speed, high resolution video-camera from which pulsation frequency and flame behavior were analyzed. Our experimental results support the hypothesis that the primary control mechanism of pulsating spread is subsurface-liquid flow, while the effect of gas-phase convection is secondary even though it governs flame pulsation. To evaluate this interpretation, two dimensional incompressible flow equations were numerically solved and fluid dynamic structure was simulated as a function of fuel-layer thickness. The calculation results explain the present and NASA's experimental results and support our proposed mechanism of pulsating spread.