A spatially one-dimensional model for the ignition of a combustible gas layer adjacent to a plane solid surface is considered. The effect of an incident laser beam from the gas side is to raise the surface temperature ; the other boundary is taken to be a poor thermal conductor in the form of an inert gaseous medium. It is assumed that the exothermic chemical reaction within the layer has a negligible reactant consumption. Three examples for the effect of the laser on the solid surface are considered : (a) a large instantaneous temperature rise, (b) a temperature jump at t = 0 which is maintained for t > 0, (c) a surface temperature variation which is a linear function of time. For (a) and (b) conditions for criticality are obtained. For appropriate states of the system, times to ignition for (a), (b), and (c) have been determined. It is shown that the theoretical results are in reasonably good agreement with experiment. The kind of physical situation envisaged might occur in a mineshaft when a stagnant combustible layer is subject to intense transient light. Although an idealization, our analysis determines conditions under which thermal runaway and subsequent explosion could take place.